Question

With the evolution of fish to tetrapods, the pelvic and pectoral girdles became successively smaller, heavier, more numerous or ribbed?

Answer 1

The question uses an incorrect understanding of evolutionary theory. Fish did not evolve into tetrapods, just as monkeys or apes did not evolve into humans. We think, to the best of our knowledge, that the ancestors of the original tetrapods were highly similar to what we see today in fish.

Now, for the question, things do not weigh as much in water, right? Would you expect the first tetrapods to be completely aquatic or would you expect them to spend at least some significant time on land? If they are on land, would their bodies be under more stress from gravity?

@sevenlions

Answer 2

should help you out. http://bosscade.com/c?r=CKUEDE4sd