There's a comic of this, which I feel illustrates this point pretty clearly
Anyways, I'm in camp that I feel Tifa's design should be modified to some degree. I always felt Tetsuya Nomura's design of Tifa was the result of being given a basic description, such as her being a tough girl who works at a bar, in which case... design wise, I can kinda see. But once we move out of this area, I always felt her continuing to wear the same outfit felt out place, especially once I became older. It's especially out of place when we get to some later cutscenes, including the infamous scene when the Weapons materialize and shoot out of the Lifestream.
I can't really comment about her breast size, since I'm not a woman and this is an area I can't really comfortable give a full opinion on, so I'll keep this brief. I feel a lot of issues probably could be resolved if designs were modified a bit, clothing wise and the like. Specifically once we move out of area where she should be theoretically be wearing more practical clothing. I do know women who have relatively big breasts, and I know the pains and struggles it is in this regards (since I know this can cause back pains, and the few friends I had over the years I met, have commented largely on this), but I do think there's a balance between having them big and practical looking, versus whatever Square did back in the day, especially with that later cutscene I mentioned earlier. So that's really all I can say.