Foreman is a big guy, and he could've hit even harder than Tyson, but prime Tyson was faster, that's how he compensated.
One thing I've few times heard is people giving Tyson props for his amazing footwork: not that Frazier missed that (you just cannot punch that well without a decent footwork), Frazier was great, but by the age Tyson turned pro, Frazier was still dealing with Olympics: by the age Frazier turned pro, Tyson was already a professional world champion: that means a lot of pro experience, and it would play a major key role, in the match.
Moreover, Tyson's defense was damn tight, so I believe it would be too much work for "Smokin' Joe".