Maybe Pete, but Aiello was on the large size. The novel says Fredo was short and burly.

I figure that FFC wanted an actor who would physically portray the relatively weak character that Fredo turned out to be. To do so, he may have wanted an actor who was rather slight in build.

To portray the Fredo of the novel, I don't recall who of the actors of 1970 could have portrayed him. Maybe Michael J. Pollard.