Not sure why you're drawing a correlation from "a lot stronger than just 8 people strong" with transferring including their quirks. Quirks doesn't equal strength. So, not sure why you're seemingly discounting the intrinsic strength of the One For All itself.
Now, I'm not saying that transferring may not be doing just that as you mention with the Quirk, but so far there has been no evidence to lead one to believe that's the case. All Might being incredibly strong is not a good enough justification to believe that.