I'm going to tell a relative that my bridesmaid was pregnant and not just fat
Why are you saying that? Is it the truth?
My relative wanted me to choose which it was.
Well, it is wrong to call other people fat. You should be nice and not criticize this woman, so you probably did the right thing.
Calling people fat will make them worried about their weight and will eventually help them lose weight.
That may be the case for some people, but others are just born with naturally heavy bodies, and they really can't control that. Even with good intentions, you probably shouldn't comment on other people's weight, as it will likely just upset them. If you have doubts about a woman, it's fair to say that she is pregnant.