It's a valid question. I've asked it myself. I have changed my diet, try to eat healthier, and while I'm moving more, I know I can exercise more. While excess weight can keep you from conceiving, it's not the root of my problems with infertility.
For starters, I was able to get pregnant while at this weight. I lost my baby at 10 weeks after going into shock. I just haven't been able to get pregnant again, which lead to months of questions, searching, and finally a diagnosis of PCOS.
PCOS is an endocrinological problem. Basically, when you have it, your hormone levels are screwed up, which can lead to diabetes, ovarian cysts, ovarian cancer, you name it. In my case, I have probably had this since puberty, as I have never had "normal" cycles. It's not unheard of for me to go 5-8 weeks between periods. I just thought I was weird. I also have excess hair in unladylike places--my husband and I both shave every morning, and if I let it go, my beard would be better than his. The reason? My body produces excess testosterone--which leads to wonky cycles, excess hair, extreme moodiness, and what else? Oh yeah, infertility. I'm on some medication to help change that, and my body is responding, but it's been another six months and still, no "good" news. I've lost some weight, gained it back, and am trying to lose it again. The weight isn't holding me back, it's the rest of it.
Would I love to be a size 12? Well, yeah. But I'd rather be happy than be worried that I might gain an ounce and not fit into my clothes. Can I lose weight? It's hard to with my body working against it. I'm not stressing about the weight, but it does bother me when random people will tell me "to just lose weight and you'll get pregnant like that". Well, no I won't, not without my medication, because my body doesn't ovulate regularly. That comment, along with the "if you'd lose 70-100 pounds you'd be beautiful", can turn my good day into a gloomy one. Do you think I chose to be the size I am? I eat less food than many of the people I know, but I still gain weight--and while I'm trying to lose, I'm doing good to lose 1 pound in 1 month. It's some, but no one ever notices, and I get discouraged. I know God loves me, and so does my husband, but it's hard to feel beautiful when you're surrounded by the message that the only way you can be beautiful is to be skinny, and that if you really want children, you'll get skinny first...I know that losing weight will make me healthier, but can't anyone be happy with who I am and then be thrilled when I make small changes?