Of course, American men are obsessed with breasts, and we can probably put it down to momism and the apron strings, or something.
As opposed to what? A "European" or "Canadian" male? "Momism", "apron strings"?
Try this one (hope I can get away with this on this board)...
Americian advertising is a very interesting thing. Whereas a nude female form seems a more acceptable, common and apparently inoffensive outside the US, my observation indicates that our advertisting leans very heavily toward positioning the female anatomy as "taboo", for lack of a better term. Seems to be much money to be made keeping it that way. I don't think this is the same for European cultures - at least if commercials and programming on TV are any indication. It certainly doesn't seem as offensive there as it appears here in the US.
I do however, believe that it is Madison Avenue that is primarily responsible for defining the subconscious responses, interpretations and "over-attention" to this particular asset.
I personally have found that in some of my own art, when I draw a female form - and consciously attempt to downplay this, I find that I'm still critiqued for the size as too large. Therefore, I can immediately determine that there is some constant reinforcement I'm exposed to regarding size that constantly shifts my scale as to small-medium and large. Advertisting was the source I discovered - then reinforced in other mediums (e.g. push-up bras that produce cleavage that makes them appear larger, etc.)
Ultimately I found if you're going to be skilled at this aspect of anatomy, you need to constantly retrain your eyes to stay more anatomically correct. However, puritan hypocritical mores we are sometimes a little too guilty of in the US tends to make this natural attribute a very unpopular subject to study. However, I can see where an artist in this field would be rather compelled to follow suit as to what seems is appropriate in print, television and movie mediums, i.e. "hey - they all got 'em", and there very well could be some marketing/editorial pressure to ensure that is the case. I mean where would good girl art be without Vampirella? I'm fairly certain part of PG's genesis had visions of Vampi floating around.
In short, "momism" and "apron strings" have very little to do with anything.