diff --git a/src/NATS.Client.ObjectStore/Models/ObjectMetadata.cs b/src/NATS.Client.ObjectStore/Models/ObjectMetadata.cs
index 4a3e94259..92b199250 100644
--- a/src/NATS.Client.ObjectStore/Models/ObjectMetadata.cs
+++ b/src/NATS.Client.ObjectStore/Models/ObjectMetadata.cs
@@ -35,11 +35,12 @@ public record ObjectMetadata
     public string? Nuid { get; set; }
 
     /// <summary>
-    /// Max chunk size
+    /// The size of the object in bytes.
+    /// It only includes the size of the object itself, not the metadata.
     /// </summary>
     [JsonPropertyName("size")]
     [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]
-    public int Size { get; set; }
+    public ulong Size { get; set; }
 
     /// <summary>
     /// Modified timestamp
@@ -53,7 +54,7 @@ public record ObjectMetadata
     /// </summary>
     [JsonPropertyName("chunks")]
     [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)]
-    public int Chunks { get; set; }
+    public uint Chunks { get; set; }
 
     /// <summary>
     /// Object digest
diff --git a/src/NATS.Client.ObjectStore/NatsObjStore.cs b/src/NATS.Client.ObjectStore/NatsObjStore.cs
index 759d0f0b1..befdca53c 100644
--- a/src/NATS.Client.ObjectStore/NatsObjStore.cs
+++ b/src/NATS.Client.ObjectStore/NatsObjStore.cs
@@ -83,7 +83,7 @@ public async ValueTask<ObjectMetadata> GetAsync(string key, Stream stream, bool
 
         string digest;
         var chunks = 0;
-        var size = 0;
+        ulong size = 0;
         using (var sha256 = SHA256.Create())
         {
 #if NETSTANDARD2_0
@@ -107,7 +107,7 @@ public async ValueTask<ObjectMetadata> GetAsync(string key, Stream stream, bool
                     {
                         using var memoryOwner = msg.Data;
                         chunks++;
-                        size += memoryOwner.Memory.Length;
+                        size += (ulong)memoryOwner.Memory.Length;
 #if NETSTANDARD2_0
                         var segment = memoryOwner.DangerousGetArray();
                         await hashedStream.WriteAsync(segment.Array, segment.Offset, segment.Count, cancellationToken);
@@ -182,8 +182,8 @@ public async ValueTask<ObjectMetadata> PutAsync(ObjectMetadata meta, Stream stre
             meta.Options.MaxChunkSize = DefaultChunkSize;
         }
 
-        var size = 0;
-        var chunks = 0;
+        ulong size = 0;
+        uint chunks = 0;
         var chunkSize = meta.Options.MaxChunkSize.Value;
 
         string digest;
@@ -250,7 +250,7 @@ public async ValueTask<ObjectMetadata> PutAsync(ObjectMetadata meta, Stream stre
 
                     if (currentChunkSize > 0)
                     {
-                        size += currentChunkSize;
+                        size += (ulong)currentChunkSize;
                         chunks++;
                     }
 
diff --git a/tests/NATS.Client.ObjectStore.Tests/ObjectStoreTest.cs b/tests/NATS.Client.ObjectStore.Tests/ObjectStoreTest.cs
index acbe86ecf..6fe6634b7 100644
--- a/tests/NATS.Client.ObjectStore.Tests/ObjectStoreTest.cs
+++ b/tests/NATS.Client.ObjectStore.Tests/ObjectStoreTest.cs
@@ -82,7 +82,7 @@ public async Task Put_chunks()
 
             Assert.Equal($"SHA-256={sha}", data.Digest);
             Assert.Equal(chunks, data.Chunks);
-            Assert.Equal(size, data.Size);
+            Assert.Equal((ulong)size, data.Size);
         }
 
         // buffer with smaller last chunk
@@ -102,7 +102,7 @@ public async Task Put_chunks()
 
             Assert.Equal($"SHA-256={sha}", data.Digest);
             Assert.Equal(chunks, data.Chunks);
-            Assert.Equal(size, data.Size);
+            Assert.Equal((ulong)size, data.Size);
         }
 
         // Object name checks
@@ -195,7 +195,7 @@ public async Task Delete_object()
         await store.PutAsync("k1", new byte[] { 65, 66, 67 }, cancellationToken);
 
         var info = await store.GetInfoAsync("k1", cancellationToken: cancellationToken);
-        Assert.Equal(3, info.Size);
+        Assert.Equal(3UL, info.Size);
 
         var bytes = await store.GetBytesAsync("k1", cancellationToken);
         Assert.Equal(bytes, new byte[] { 65, 66, 67 });
@@ -207,8 +207,8 @@ public async Task Delete_object()
 
         var info2 = await store.GetInfoAsync("k1", showDeleted: true, cancellationToken: cancellationToken);
         Assert.True(info2.Deleted);
-        Assert.Equal(0, info2.Size);
-        Assert.Equal(0, info2.Chunks);
+        Assert.Equal(0UL, info2.Size);
+        Assert.Equal(0U, info2.Chunks);
         Assert.Equal(string.Empty, info2.Digest);
 
         // Put again
@@ -622,4 +622,45 @@ await Assert.ThrowsAsync<NatsObjNotFoundException>(async () =>
             await b1.GetBytesAsync("name1", cancellationToken: cancellationToken);
         });
     }
+
+    [Fact]
+    public async Task Metadata_field_types_match_spec()
+    {
+        var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));
+        var cancellationToken = cts.Token;
+
+        await using var server = await NatsServerProcess.StartAsync();
+        await using var nats = new NatsConnection(new NatsOpts { Url = server.Url });
+        var js = new NatsJSContext(nats);
+        var obj = new NatsObjContext(js);
+
+        var store = await obj.CreateObjectStoreAsync(new NatsObjConfig("b1"), cancellationToken);
+
+        // Test that Size property is ulong (uint64 in NATS spec)
+        var metadata = new ObjectMetadata { Name = "test" };
+        metadata.Size = ulong.MaxValue; // Should compile without error
+        Assert.Equal(18446744073709551615UL, metadata.Size);
+
+        // Test that Chunks property is uint (uint32 in NATS spec)
+        metadata.Chunks = uint.MaxValue; // Should compile without error
+        Assert.Equal(4294967295U, metadata.Chunks);
+
+        // Test with actual object metadata from store
+        await store.PutAsync("k1", new byte[] { 1, 2, 3 }, cancellationToken: cancellationToken);
+        var info = await store.GetInfoAsync("k1", cancellationToken: cancellationToken);
+
+        // Verify Size is ulong
+#pragma warning disable IDE0007
+        // ReSharper disable once SuggestVarOrType_BuiltInTypes
+        ulong size = info.Size; // Should compile without error
+#pragma warning restore IDE0007
+        Assert.Equal(3UL, size);
+
+        // Verify Chunks is uint
+#pragma warning disable IDE0007
+        // ReSharper disable once SuggestVarOrType_BuiltInTypes
+        uint chunks = info.Chunks; // Should compile without error
+#pragma warning restore IDE0007
+        Assert.Equal(1U, chunks);
+    }
 }
diff --git a/tests/NATS.Client.ObjectStore.Tests/WatcherTest.cs b/tests/NATS.Client.ObjectStore.Tests/WatcherTest.cs
index ff4268c53..73fa43f0f 100644
--- a/tests/NATS.Client.ObjectStore.Tests/WatcherTest.cs
+++ b/tests/NATS.Client.ObjectStore.Tests/WatcherTest.cs
@@ -30,7 +30,7 @@ public async Task Watcher_test()
                 {
                     count++;
                     signal.Pulse();
-                    Assert.Equal(count, info.Size);
+                    Assert.Equal((ulong)count, info.Size);
                     if (count == 3)
                         break;
                 }