A pig cannot grow beyond a certain point because its skeletal structure would be unable to support its weight- and if pigs were selected to grow a radically different skeletal structure, they could grow much larger- but such a change would take millions of years (perhaps less with cleverly implemented artificial selection).
They don't need radical changes in structure, same structure only bigger, thicker and longer leg bones. There are decorative pigs that are no bigger than 50 lbs, and there are big ones 400 lbs ones, both have same skeletal structures, only difference - size. So if 50 pound one can be made into 400 pound one why stop there? Lets go to 40000 lbs. After all, elephants are bigger than pigs and they don't have radical differences in skeleton structure from pigs.
Elephants do have quite different bone densities, and yes, the proportions of their skeletal structures are quite different. This in no way proves your point. These are not limits placed upon evolution by its own nature, merely by the laws of physics. However, the laws of physics say little about generation of novel genetic information, structures, and functions.
A fly cannot resist impacts with a certain amount of force because the materials it is made of possess a certain inherent strength, determined by their chemistry.
Did you just say that limit of fly's change is determined by the chemistry of their bodies? Sounds like a limiting factor to me, does it not?[/quote]
Yes, but it is not a limiting factor evolution imposes on itself. None of your examples have anything to do with limits prohibiting changes of "kinds".