Yes, mountain bikes definitely size differently than road bikes. You need a shorter frame for nadular clearance and so the top tube isn't limiting the angle of the bike.

For a guy 6'1 a 21" frame is perfectly reasonable. The number I look for is the top tube length, it's a virtual number since the thing is sloped, but about a 23" top tube works for me. You're mileage will vary, but you see what I mean. The seatpost can be as long as it needs, since on an MTB you're out of the saddle for any hard pedaling.