I mean, if you actually weighed Volgin and Tifa, there'd be a huge difference. Tifa would probably weigh about 120 lb/60 kg or so, at the most. Volgin, on the other hand, would be at least twice that. He's far taller than Big Boss, who's hardly a midget.

In-game battle mechanics really need to be left aside from this kind of discussion, since they naturally defy reality: for example, Big Boss surviving countless bullet wounds and explosions throughout MGS3's gameplay, including shotgun blasts to the face or groin, when we know that he "really" lost his eye to the muzzle flash of a single bullet, as shown via a cutscene.

I argued in my first post that Tifa could theoretically beat Volgin, via her skill and the unnatural strength she exhibits in the FF world. We know for a fact (via Dirge of Cerberus) that she can take on gun-wielding soldiers and prevail. However, 'ability to win a fight' and 'strength' aren't the same thing. If both characters existed in the same fictional universe (instead of different worlds where different rules apply), then Volgin would be far stronger.