Heavier poundage doesn't make you shoot faster arrows--lighter arrows do. If you are shooting 10 gpp, then you'll get the same trajectory from a 30# bow as a 70# bow, all else being equal.
However, as noted, you may be getting a better release with the heavier draw weight, which will give you better performance. One of my demons is my release, and I do better with heavier draw weights.
Most all the good shooters I know/know of shoot 50# or less. Rod Jenkins usually shoots bows in the low 40's.
Proper tuning, proper nock fit, etc. will also pick up a fps here and there, but my guess is you are getting a better release and that's where the biggest difference is, unless you are using more gpp with the lighter bow.
Chad