Convince partner to gain weight.

My girlfriend is really beautiful face wise. She’s very health conscious which is good because we’re both active gym members. My problem is that she’s really into the slim model type physique”Victoria beckham type” but me being black I love for a girl to have curves.the more the better.i really love her and constantly compliment her but I never tell her the truth that I’m just not into skinny girls. I fear that one day I’ll stop being attracted to her and my desire for a girl with curves would get too strong and ruin our relationship. How can I express this to her without upsetting her and how can I encourage her to gain weight. Or should I just learn to live with it?