HierarchyObjectSize.cs

using System;
using System.Reflection;
using UnityEditor;
using UnityEngine;
 
[InitializeOnLoad]
public class HierarchyObjectSize
{
	static HierarchyObjectSize ()
	{
		EditorApplication.hierarchyWindowItemOnGUI  += (instanceID,rect) => {
			DisplaySize (instanceID, rect);
		};
	}


	static void DisplaySize (int instanceID, Rect rect)
	{
		rect.x += rect.width - 50;
		rect.width = 50;

		GameObject go = (GameObject)EditorUtility.InstanceIDToObject (instanceID);

		int bytes;
		try
		{
			bytes = Profiler.GetRuntimeMemorySize (go);
		}
		catch
		{
			bytes = -1;
		}

		string text = EditorUtility.FormatBytes (bytes);
		GUI.Label (rect, text);
	}

}