Fifth wheels feel larger because of the higher ceilings. In many Class A's the front driver area is wasted and even if you use the seats you still have a couple of feet that is not used which could be a large closet or something else. In addition, with higher ceilings the slide outs are also taller really opening up the interior. The height differences can really make a big difference. When your slide out height is taller than most Class A's ceilings it really opens up the room a lot.
So on average fifth wheels have more diverse floor plans, feel larger and apples to apples tend to be a bit larger on the inside than a MH.
MH's have other advantages. To each his own.