It’s hard to ask your partner to lose some weight. But don’t make it personal! Weight is a sensible topic for everybody, so when you ask your significant other to lose some weight it could bring you some complications. Be nonchalant about it!
Just offer a healthier diet for the both of you, sign on a gym together, go run, or go grocery shopping together. Make it couples issue not his/hers.
Also some motivation never hurts. Tell him/her they are looking beautiful and encourage them to have a healthier and better life.