Yorks,
That would be my (hiterto ignored) point. I'm not that large (12 - 13st) and I would guess my erstwhile instructor was about the same. I can't remember whether fuel is forward or aft of the normal CofG range - that is, whether the situation is worse with full or nearly empty tanks.
Either way, the PA28-161 is operating very close to its limits with no ballast (walking or otherwise) in the back. This never stops them instructing/testing in them though, does it?
TPK