Consider allocatedBytes() instead of bytes() in Storage{Buffer,Memory}.

src/Storages/IStorage.h

@@ -449,14 +449,18 @@ class IStorage : public std::enable_shared_from_this<IStorage>, public TypePromo
     }
 
     /// If it is possible to quickly determine exact number of bytes for the table on storage:
-    /// - memory (approximated)
+    /// - memory (approximated, resident)
     /// - disk (compressed)
     ///
     /// Used for:
     /// - For total_bytes column in system.tables
     //
     /// Does not takes underlying Storage (if any) into account
     /// (since for Buffer we still need to know how much bytes it uses).
+    ///
+    /// Memory part should be estimated as a resident memory size.
+    /// In particular, alloctedBytes() is preferable over bytes()
+    /// when considering in-memory blocks.
     virtual std::optional<UInt64> totalBytes() const
     {
         return {};

src/Storages/StorageBuffer.cpp

@@ -797,7 +797,7 @@ std::optional<UInt64> StorageBuffer::totalBytes() const
     for (const auto & buffer : buffers)
     {
         std::lock_guard lock(buffer.mutex);
-        bytes += buffer.data.bytes();
+        bytes += buffer.data.allocatedBytes();
     }
     return bytes;
 }

src/Storages/StorageMemory.cpp

@@ -169,7 +169,7 @@ std::optional<UInt64> StorageMemory::totalBytes() const
     UInt64 bytes = 0;
     std::lock_guard lock(mutex);
     for (const auto & buffer : data)
-        bytes += buffer.bytes();
+        bytes += buffer.allocatedBytes();
     return bytes;
 }