I'm going to tell my friend that it's funny how his girlfriend is skinnier than mine but has much bigger boobs
Have you been waiting to tell him this?
Yes, I've always wanted to tell him this
It is normal to sometimes make comparisons in your head but you shouldn't try to make comparisons about your partner. It will make you feel dissatisfied with them and cause problems.
I don't mind. Do you have a problem with this?
It's not really my place to have a problem with this, but it is not polite to compare people's bodies especially when they didn't ask. People can be very self-conscious about their appearance so you should be sensitive about how people feel.