HPROF file format does not account for object headers and object/array alignment(s) This makes instance size calculations approximate. While it is okay to approximate for alignment, not accounting for object size is a problem. For example, for java.lang.Object instances (or of subclasses that add no instance field) and zero length arrays, the histogram and other size calculation queries (sizeof built-in in OQL) report 0 as size. If there are enough zero-sized objects, histogram gets distorted. In a simple test case, many instances of a class Foo with no instance fields were created. Because of the 0 size, histogram shows Foo down the order. It is better to account for object headers -- 2 pointers (one for klassOop and one for markOop) are used in hotspot JVM. Most VMs use two or more pointer sized object header words. So, it is better to have minimum object size to be size of 2 IDs (or pointer). This will improve accuracy of instance sizes.