The documentation said it\'s used for limit the size of the output object, but based on my test, it\'s not what I expected:
0:000> !dumpheap -mt 000007fee
WinDbg's default number format is hexadecimal. 0x400 is 1024 and 0x399 is 921, that explains the output. If you want decimal numbers, use the 0n prefix, e.g. 0n400 and 0n399.
0x400
0x399
0n
0n400
0n399