diff --git a/core/trino-main/src/main/java/io/trino/operator/MultiChannelGroupByHash.java b/core/trino-main/src/main/java/io/trino/operator/MultiChannelGroupByHash.java
index 6c4f6e20d6c2..5c91eaaa8324 100644
--- a/core/trino-main/src/main/java/io/trino/operator/MultiChannelGroupByHash.java
+++ b/core/trino-main/src/main/java/io/trino/operator/MultiChannelGroupByHash.java
@@ -174,7 +174,8 @@ public long getEstimatedSize()
                 currentPageBuilder.getRetainedSizeInBytes() +
                 sizeOf(groupIdsByHash) +
                 sizeOf(rawHashByHashPosition) +
-                preallocatedMemoryInBytes;
+                preallocatedMemoryInBytes +
+                (dictionaryLookBack != null ? dictionaryLookBack.getRetainedSizeInBytes() : 0);
     }
 
     @Override
@@ -543,6 +544,7 @@ private int registerGroupId(HashGenerator hashGenerator, Page page, int position
 
     private static final class DictionaryLookBack
     {
+        private static final int INSTANCE_SIZE = toIntExact(ClassLayout.parseClass(DictionaryLookBack.class).instanceSize());
         private final Block dictionary;
         private final int[] processed;
 
@@ -572,6 +574,13 @@ public void setProcessed(int position, int groupId)
         {
             processed[position] = groupId;
         }
+
+        public long getRetainedSizeInBytes()
+        {
+            return INSTANCE_SIZE +
+                    sizeOf(processed) +
+                    dictionary.getRetainedSizeInBytes();
+        }
     }
 
     @VisibleForTesting