Yeah, very very very very important. Most important thing in the world.
j/k heh
Well, for me it's not a so important aspect. In the physical appearance department, I prefer cuteness and aesthetics over everything else. Also, if I had to choose from slimness or breasts, I would choose the former. I wouldn't complain if a girl has them big, but I wouldn't complain if she has them small.
As long as she is pretty and cute, everything is fine.