Relationship between physical strength and punching power

Generally that's true. But after 175-180ish, it starts to get a little blurry regarding weight and punching power. You may have a power punching light heavy or cruiser that hits harder than some heavyweights. For example, most people accept 185 pound Rocky Marciano as being a harder puncher than 215 pound George Chuvalo or 180-190 pound Jack Dempsey as being a harder puncher than 210 pound Muhammad Ali.

On the other hand, you have a 210 pound Ali who was as fast or faster than a lot of middle weights and welterweights. Although it's the norm for smaller guys to tend to be faster than bigger guys and for bigger guys to be stronger and/or hit harder than smaller guys, that's not always the case.