The comparison is based in at least two real-life circumstances:

Moretti is alleged to have muscled studio chief Harry Cohn into giving Sinatra the part of Maggio in "From Here to Eternity," which re-launched his career. (As it happened, Cohn had originally selected Eli Wallach, but he wasn't skinny enough for the part, so Sinatra got it. But the Moretti story took hold.)

Also, Moretti, a feared killer, was Frank Costello's "strength," as Joe Bonanno put it in his autobiography. And some here, including me, believe Vito was modeled after Costello.