I have found that sitting down not only spreads my backside from side to side, but about 5 inches top to bottom. I found this out by securely holding my tape measure at my natural waist, tying in several places to my leg with string, and holding the top and bottom of the measure in pinched fingers. As I sat down, I let the end of the measure behind my knee slide between my pinched fingers. It moved five inches.

Bigger guys spread more in all directions. When you sit, and your kilt is to short, it is because your big ham hocks have spread out and what was once hitting the back of your knee is now lucky to be hitting your mid thigh, no matter how much you sweep there is no way to make your kilt longer to account for the spread. As your leg bends, the length between the back of your knee and your waist grows along the curve of your buttocks, doubly so when you sit.

This also explains why troosers give big fellas wedgies when they sit down. The crotch depth (A subject that I actually just took a test on heh) does not account for spread or fill. Or when you bend over and your pants rip.

For guys with guts, double wammy. When you bend your torso, your gut presses down on your lower abdomen, causing quite a bit of spread and forcing your body to move to other locations now that mass is pressing in on the locations where the jiggly masses previously were.