As a gay man, I have 0 personal experience with women's breasts. However, make sure that 'painful' doesn't translate into 'physically injurious'. For large-breasted women, improper breast support can lead to serious problems such as strained backs. I'm not saying don't do it, but I'm reminding you that a dom's first obligation is to make certain that what he does to his sub won't be harmful. So make sure you understand the physical limits involved.