For me it's mainly due to the money factor. I'm talking agency girls in $400-600 range or more for western agency girls.
If it's a free punt most of us likely take the average 5/10 girl and say, nice! But since I'm not a millionaire I kinda wish the girl look 7/10 at least with good service after giving up my hard earned money.
Breast size actually not a big concern for me unless it's really tiny lol