According to you for what arcane mystery terrific right hands can start from Wilder's pencil legs which were literally destroyed by Fury in the second fight?
The notion that power comes from the legs meaning that they should be big and strong is a silly and misguided one. It's all about the rotation of the shoulder and hips, as well as back strength and punching form. Some lightning speed, long leverage-giving limbs and big fists tends to add to it as well. Having a good base with the legs is a necessity of course but they don't need to be chunky for that, just set in proper position for balance.
Power comes from the legs through the hips. Good balance and push is essential. That being said there are exceptions such as Foreman.
It is baffling how he generates so much power. It's clearly his upper body, hips, shoulders and punching technique.
Skinny legs can be really powerful. Think about basketballers, high jumpers, etc. Some of them have really skinny legs. But those legs must be powerful, otherwise they wouldn't be able to jump like they do.
wilders puny legs only make him punch harder cause he can barely support his own weight, thereby shifting his bodyweight heavily into his punches
Wilder's very springy and quite fast for a big man. All his power comes from being able to quickly leap forward and fling his entire weight into his right hand straight down the pipe. You don't need to have thick legs to do that; in fact it might actually be a disadvantage to the way Wilder throws power shots (which in his case is basically just the straight right hand).
So much of punching power comes down to STYLE instead of this "born puncher" notion. Wilder commits 100% to his power punches. He steps IN to them, and leaves himself off balance. I think a lot of this has to do with the fact that he never learned to box properly. Trainers teaching a 10 year old amateur will put an emphasis on throwing a punch or combo and immediately engaging defense when the last punch is complete, which requires you to be balanced at the transition. Wilder isn't balanced to transition to defense and doesn't seem to care about that. His entire style is going all in on a lunging 1-2 and counting on his opponent to either shell up or fall down and not counter him. Anyone employing this style will get more KO's, and will also be KO'd more. It worked for Wilder against lesser opposition as he has the height and snap to close huge distance. Natural power plays a part, but this is much more of a style thing. In a sentence: Wilder packs a helluva punch because that's all he knows/attempts/wants/can do.
Bigger legs aid punch resistance. As for punching power, well Bronze Bomber and Hitman have shown you can bang with thin legs.