Why did my ex get a new girlfriend that is everything he did not want me to be?

I was in a relationship with a Guy for over 5 years, lovely guy, But we became more friends than lovers and he had the guts to end the relationship.
We are now 6 months later, still friends, I have a new boyfriend he has a new girlfriend recently

. During our relationship he has always told me two things were wring with me:

1) i had to to gain a little bit of weight. I am quite fit, not thin, but I am naturally petite, And can't seem to gain weight. This has always made me feel a bit insecure. He admitted to Like girls with a bit more fat on their bodies, up to obese.

2) Also, I am open to adopting a more sustainable lifestyle. I wouldn't cut meats And diary out of my diet completely, But i Like tasting different foods. He always got very Angry when I tried this, really annoyed that I Maybe would Cause problems with this If would go out for dinner together.

Now the new gf: she is an extremely thin fitgirl, everything that he always accused me to be: too muscular/thin And a fuzzy eater. He can't even take her out for dinners because she won't eat anything from the menu in restaurants.

Now I'm confused, we had two big discussions in our relationship;
1) him making me feel insecure with saying that I need to gain more weight And 2) diet.

And now the new girlfriend is extremely ticking those two boxes? How can you change this fast as a Guy? I thought our break-up went quite smooth But then this confuses me.
Why did my ex get a new girlfriend that is everything he did not want me to be?
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