I don't change the Str modifiers, but I change the size modifiers to carrying capacity.
It lets strength by size be regularized. Assuming that dead-lift strength is proportional to cross-sectional area, DL ~ d^2, and the size modifiers by size category is x2 for each size larger (same as it works now) and /2 for each size smaller (different than now, since Small is x3/4 in the PH), then Str ~ d so for creatures of a similar build, doubling size (increasing category by +1) doubles the Str score. Generally, of course, the same build won't be feasable with a larger creature, so the Str scores won't quite double.