Well, if EXR-5 has 8 Mb and PSR2100 has 32 or more... And, if EXR-5 has more sounds than PSR2100 (so, has less Mb for each sound)...
then, how is possible EXR-5 has better sounds (as Fran say).

I think there is something not clear about Mb for sounds.
For example, 2100 has two times the 2000 Mbs, but does not sound a lot better, only a few better, if something.

Perhaps Yamaha Mb are compressed... or Yamaha counts its Mb nor only for sounds... of Roland make better sounds, or ...