I'm NOT particularly busty and I often have this problem, even when a shirt has quite a bit of positive ease. For me, it's a combination of two things. First, I have a very short upper body, so the buttons are not quite in the right place vertically. For a perfect fit, I'm told that you have to have a button that lines up exactly with the widest point of your boobs, which is usually at nipple level. Plus the armpits tend to be too low, which also causes weird pulling sometimes. Second, my boobs are very wide-set and I have a broad sternum, so there's this wide flat gap between them that seems to exacerbate the pulling problem.
Petites CAN work, but generally I end up with sleeves that are too short then. Honestly, my solution is just to sew up the button plackets in the offending area and just put them on over my head.