HPROF_GC_CLASS_DUMP subrecord in hprof heap dump contain "instance size" field, but the value is VM-independent and does not reflect memory size actually occupied by objects in heap.
There were requests to improve accuracy of the value (JDK-8005604, JDK-8176520), but change of the memaing of an existing field is considered unacceptable.
HPROF_GC_CLASS_DUMP has 2 "reserved" fields, but their size are "id", also subrecords in HPROF_HEAP_DUMP/HPROF_HEAP_DUMP_SEGMENT are supposed to be VM-independent.
To provide information about actual size of the objects in heap new top-level records can be introduced.