My youngest son was the spitting image of Yone when he entered high school. By the time he was entering college, he was 6'5" and a lean 190 pounds. He went from cute chubby kid to tall handsome man in the span of a couple of years. So, I'm with you. Not all overweight kids, especially males, stay overweight. They grow out of their "baby fat" stage and it is used as fuel to propel them into their adult form. My husband was exactly the same. Cute chubby kid in high school and by the time he entered the Navy, and I met him at age 19, he was 6'2" and 170 pounds. Both my husband and son are powerfully built and strikingly handsome and both were ridiculed in school for being the "fat kid". I even told my son exactly the same thing others told your boyfriend (ex?). That when he was older, he would not look this way because of his similarities to his father. He often came home in tears and wanted to diet. It killed me. He was a growing kid and he was trying to starve himself. So, we brought out all of my husband's old pictures. My son thought they were pictures of him! LOL I told him to not worry about dating until he was in college. He said okay and once in college he began dating. Most of his girlfriends were exceptionally attractive. The funny thing is, the girls who wouldn't look at him twice in high school, now won't leave him alone. Needless to say, he doesn't have a whole lot of time for them.
I think people who "grow into" their attractiveness, but we're once ridiculed for being "less than ideal" when they were young, tend to be more humble and appreciate the beauty in people that can't be seen with the naked eye.
You did it again. You are the one equating the word "fat" to unattractive. Fat is defined as:(of a person or animal) having a large amount of excess flesh. Do you see the word unattractive there? I don't. Raindragon claims her ex "had no idea he would become attractive", but neither he, nor she, claim he was unattractive to start. There are times when a person must read between the lines to understand what is being said. Then there are times when we must take people at the words they say rather than putting our words in their mouths. In no other way does Raindragon imply that she found her ex's weight in his youth to be unattractive. She says he was fat, ie: he had an excess of flesh on his body. She is stating a fact. She makes no mention of her opinion of his former appearance. The only opinion she states is on his current appearance, which is that she found him attractive. Now, she quotes HIM as saying that he had "no idea he would become attractive", perhaps meaning that he, himself, felt unattractive in his former appearance, but her opinion is never mentioned. Basically, you have seen an insult where none existed and that caused you to put words into Raindragon's mouth. You have projected your own prejudices or insecurities onto Raindragon's comment.
You even go as far as to insult me, possibly because you assumed I had insulted you first. Since it is difficult to infer tone and intent in the written word, I won't take offense at your blatant insult. However, I will assume that you are accustomed to being insulted and lashing back. I understand and you have my sympathy, but you also have my respect. I normally wouldn't take an insult without a fight either.
That said, rather than falsely accusing someone else of being prejudiced or assuming an insult, you may want to use that energy to examine your own views, words and prejudices. I have studied human psychology for nearly 30 years and you, my dear, are a classic case of projection and a mild case of persecution complex. Though, admittedly, spending time on the internet can give anyone a case of persecution complex. So, let's discuss projection. You project your own prejudices or insecurities onto others because you don't want to admit that you have them. Raindragon is not the one here who finds a person being overweight unattractive. That is you, but you are too frightened or ashamed to admit it. Maybe you, or someone you love, is overweight and that fact bothers you. You desperately want to be rid of the feeling that they, or you, are unattractive, but you have yet to succeed. It is okay to feel this way, but if you truly want to be rid of a prejudice or insecurity, you must first admit that you have one. When you deny your feelings, you are left unable to do anything with them, and they fester. You are not a bad person for feeling this way and you're not even a bad person for projecting your denied feelings onto Raindragon. You're human, we all have faults and flaws and you have done nothing that is unforgivable. I genuinely recommend speaking with a doctor, nurse or counselor - preferably a counselor - who can help you come to terms with your feelings and then offer you ways of changing them.
I am not saying any of this to insult, degrade or belittle you and I apologize for seeming harsh. I am attempting to give you sincere advice and consultation with good will. Free advice is worth every penny and my advice is free. So, you can take it or leave it, but I offered it in good will with the hope that you could releive yourself of a burden and live happier. I wish you the best of luck and I hope that things work out well for you.

I went out with a very attractive man, very very attractive. He showed me a picture of himself as a kid. He was really fat. He said that he had no idea he'd become attractive, "but they keep telling me," he laughed.
He simply outgrew his fat, round child self. It's natural for some people, so I wish people would stop saying they didn't like that he lost weight. It's natural. I can accept the story as it is because I've seen that guy in real life, and he grew up to be super attractive.