I don't think measuring big means the baby is always big. There is so much more to it than that, as others have pointed it. If you did have polyhydramnios, that is a serious serious condition. It can be a sign of birth defects sometimes, but it can also happen for no reason.
Mothers and babies with polyhydramnios are at a higher risk for complications such as placental abruption, cord prolapse, and perinatal death. It depends a lot on the severity of the condition, however. Sometimes it's just a mild or moderate increase and all you need to do is watch things carefully and everything is fine. BUT - if you have not been monitored for this, I think it's important that you do.