I think you're seeing Zoan abilities different to how I am. As shown by Lucci and CP9, Zoan act as a multiplier to the user's strength. By which I mean if your punch does 100n force, when you go into Zoan mode, it'll do 300n force. Again though, as CP9 have shown through their power rating thing, a multiplier doesn't matter if you don't have the strength to begin with. I mean you could give Usoppu (pre TimeSkip) a Zoan ability, but his punch strength is only 5n force, so having a 3x multi would give him 15n force, which doesn't compare to the 100n force of Lucci. These are examples, I'm just using random numbers, but I think you get the idea. WB was a beast, look at the size of his guns (muscles). There are people who train for Strength and those who train for speed and WB was clearly the former. If his ability wasn't Quake but some Giant Zoan, his punch would have taken out Akainu through brute strength alone, that that would have been imba.
The fact that in OP world, a human with no df whatsoever can still be physically stronger (ex Rayleigh, Zoro) than most zoan types. And even those whose df doesn't increase the user's brute strength is still stronger than any Zoan type we've seen (ex.WB). So normal Luffy dealing damage BB's zoan body is not far-fetched (though he still needs to increase his physical strength or stronger CoA haki imo).