I think too big would be when they start to cause problems for the person they are attached to. I wouldn't wish discomfort on someone just for more boobs. But maybe that makes me not quite selfish enough...
My wife is a 32G...I think the line is when the woman feels comfortable showing them. The only reason she's a 32G is because she's put on a little weight *she weighs 160 lbs now i think?* So she doesn't feel as confident being naked.
I on the other hand feel completely confident when she's naked