I'd sweat buckets too if I had to carry around five-pound fleshbags on my chest...

Can you imagine what running with these would be like?

She did, in the anime and there was no problem, she was fast!!!
Those breasts must bring some aerodinamic to her body.

It´s noted in the novel, that her breasts strain her back and shoulder.

I love this about big-breasted women: It gives you opportunity to come closer to them, by offering a good back massage. XD