What to do past 9ozs of a bottle

Hi, I'm wondering if someone can provide some advice on bottle feeding sizes and when is best to wean. I have twins one is on 8ozs other is on 9ozs. im wondering what happens when 9ozs no longer fills my babies. I use Tommee Tippee bottles which hold 9ozs and from what I can find I think these are the largest bottles Tommee Tippee makes? Therefore once my baby is no longer full from 9ozs is it then time to wean as they will need more than 9ozs if that makes sense. My twins are 21 weeks old. Any help\advice would be appreciated thanks.