I think in general women's weight is made much more a deal of than men's, which is why we see some of this "backlash" from TV shows on Lifetime to stores like Lane Bryant actually using models who fit into their clothing. I personally know men who are fat-- and I don't mean 10 or even 20 pounds overweight-- who don't really care about their weight and/or don't see themselves as heavy as they are. I think we tolerate more variety in mens' shapes and sizes than women's. This is changing and now men are under more scrutiny than ever, but still not as much as women.
As far as clothing, my husband was a 40 about a year ago and we never had trouble finding clothing for him in stores. I've seen 42s and 44s in stores regularly, and sometimes up to 50.
Clothing is marketed to those who will spend the most money on it, and from what I can tell, young, image conscious women buy the most clothing (and tend to be thinner) and men in general buy less and have fewer choices in fashion. I run into this with my son and husband both-- a store like Old Navy, for example, is 75% clothing for females and 25% for males. I think this is because men simply buy less. I would also think that men who are larger care even less about clothing than most men do, and that stocking a lot of larger sizes doesn't make money.
With all that said, it is nearly impossible to shop at most stores if you're a woman over a size 14 or 16. Last year I was a size 22, and I had the choice of shopping in miniscule plus size departments or a few specialty stores like Lane Bryant. I celebrated when I hit a size 16 because I could suddenly buy clothing at most stores. As for a comparison, I'd say a size 16 and a size 40 are about equivalent between men and women. Those are both sizes where a lot of stores stop stocking clothing.
I used to get really annoyed because I felt that clothing manufacturers thought that fat women all want to look ugly or don't care about their appearance. If you look around, though, it's kind of true. I know for myself going into a fitting room and trying on clothing was a nightmare because I had to look in a mirror and be reminded of how fat I was. I simply didn't want to deal with my appearance, and I think most women in larger sizes feel the same way-- so they buy less and want to spend less. There's also a feeling among some women who are plus-size that they are going to take off the weightso they only buy what they feel they need. Some of the more moderately priced stores like Old Navy, Wal-Mart, Target, and Kohl's carry extended and larger sizes because I imagine some market research has found that women in a size 18 are less likely to pay $80 for a pair of jeans than a woman who is a size 6. So, even though size 16 is average, I think most women at that size don't buy a lot of clothing which is why it's harder to find.
I wished it wasn't the case when I was in plus size clothing, but people who see themselves as fat (which most average women do) just don't feel good about themselves so they don't spend as much money on looking good because it feels futile. After all, fat equals ugly in our society. It sucks. But I think it's why it's hard to find clothing if you're a size 14-16 or up. I think it's similar for men except that men in general buy less clothing than women.