How do I fix this

So i was talking to my bf and honestly he doesn't trust in relationships but he's given me his trust. But i wanted him to open up so i asked him about his ex. He opened up to me about how relationship went and all that and well it was fine til i saw her profile picture.

Like she's a very pretty girl and she's skinny. Which i have no problem with her being pretty because i think I'm pretty myself but I did feel a way about her being skinny. Which lemme explain.

Honestly it's just my own insecurity but my bf encourages me to workout and go to the gym and sometimes it's a nice push but sometimes it feels like why do you keep asking me if I've worked out? Like you knew i was fat when you met me so why try to change me now?

And then i was just asking him if it was a difference between us and he said he doesn't know. Just that i made him happy and that he could actually trust me.

Then he said the he will say that the thing he liked about both of us was that we both have a big butt. Which idk if he meant it to come out that way but honestly that just made me insecure.

And then he was about to say "i will say she knew how to cook so good..." And honestly i kind of snapped because i told him that if you like her that much then go back with her then. Because she's skinny and you always want to me to lose weight like you didn't know i was fat the first time we met. And he keeps trying to get me to learn how to cook because I'm more of a spender, and well i know how to cook a few things but I'm not interested in that. So i got mad when he said that cause it all made sense about all these things he tells me. I told him to stop trying to turn me into her.

He said i was crazy and he shouldn't have opened up and i know i went off on my own insecurity but honestly I've been feeling this way about the gym thing for a while. I'm not an unhealthy person to the point where he's saying that cause he's worried about my health. I just feel like why do you keep encouraging me to go to the gym so much like you knew what i looked like when we met.

Please be nice.