An interesting theory alex c, but one that is not entirely supported by the context in which the line has been used. For example, John Hurt (1928)sings:

Raise up, baby, get your big leg off of mine.
Raise up, baby, get your big leg off of mine.
It's so heavy, make a good man change his mind.

Roosevelt Sykes, in his 1933 recording Big Legs Ida Blues, sings:

Mmmm, Ida, you know you got a aggravatin' shape.
Oh, Ida-ho, you got such a aggravatin' shape.
You know, it shake just like that jelly shakin' on the plate.

And more generally on the appeal of larger women, Tommy Johnson sings, in his 1928 Big Fat Mama Blues:

Cryin' big fat mama, meat shakin' on her bone.
Time her meat shake it's a skinny woman lose her home.

These examples would suggest that (whatever the sexual morals of the women concerned) 'big' in the context simply means 'big'!

all the best
Brian