Very confused on cropping!! Related to HS10

HS10 gives you a image 3,648 x 2,736 and focal length is 720mm. The lens on HS10 is actually a 126mm f5.6 lens.

Imagine if this lens can be taken off HS10 and put into a 5D. It will give a 4368 x 2912 and focal length is 126mm. We can crop it using PC to emulate the 720mm image that HS10 give. To do that, we need to divide 4368 X 2912 by 5.7. We will get 766 X 510. Won't this dimension not even meet the minimum recommended dimension to print a 6X4 photo?

I understand the theory that a lens will always be a lens, no matter which camera you use it. What am I missing out here to complete my understanding?