I think the main factor is that the average chisronos of girls (age and weight) are more significant that the average chisronos of guys, which are usually changeable (bread earning capability, maturity, etc). I know a few guys married to girls ~10 years younger than them (I'd have to marry a 10yr old to do that), but not a single couple vv. Same with weight.
The result is that there are far more boys in the 'prime category' than girls.