Bra sizes.

Jess👑 • She wears strength and darkness equally well.
Can somebody explain to me. So I have been wearing a 36c, but it seems to big for me and a 34c is to small. So I recently went to go get sized at Victoria Secret and told  I was a 34d Bc the 36 c was to big. But I though a size D was bigger than a C? I'm sorry if I sound dumb. I just need explanation lol