What causes insecurity around boyfriend?

So im really insecure around my boyfriend especially him. I’ve always be insecure but it’s starting to affect our relationship whenever he compliments my body I think he’s lying just to be nice. I’m skinny and want to be bigger so bad. My biggest insecurity is my breast size which he lies on daily and makes me so much worse. I rather have a guy say they’re small but I love them, versus oh they’re so huge biggest ever! It makes me so upset I can’t take it. I get in talks telling him be honest about my body it’ll help me be confident and he still goes on saying he isn’t lying when he is I haven’t ever showed him my boobs we been together two years and I don’t like wearing no bra around him or anything I always have a shirt on during sex and hardly wear bathing suits around him and we live together so it consists of me AWAYS wearing a bra. He says oh I know they’re big bc they pop out in shirts and all which is called a push up bra...I’m only 21 and have tried everything pills cream so much to make them grow and don’t like the insecurity I feel only around him... Is this a common thing or what?