7 1/4 why weak?

I know some people will swear by the 7 1 /4, maybe all the ones I've had were already woreout when I got them!? just wonder if anyone can give some info why it is weak? I got a couple in early A slant cars i'm working on.
I 've had many B bodies and few A bodies with the tried and true 8 3/4. can anyone give some reasons why the 8 1/4 is a good rearend, other than obviously a little bigger!????? thanks for any input.....