I think that you should definitely talk to your coach before you leave. Ask if you can meet with her before or after practice and have a discussion. Explain your difficulties and that you don't think you can keep your flyer safe. Your coach may not put you in as flyer - she may put you in another position that's better off than backing, but still gives this girl a chance to fly. I'm sure if you talk to her outside of practice she'll realize the issues you're having and try to figure something out with you.
On another note, your flyer is NOT twice your size. If she was twice your size, she would weigh 146 pounds. Think about it. 14 pounds is not very much weight. But it isn't about her weight, weight isn't all that matters. I understand that you don't feel like you can back, but your coach may think you're just not trying because you want to fly. That's how it comes across when you say "my flyer is twice my size" and "I got taken out". Maybe you should make sure you're trying to get stronger and try really hard so that the coach sees that you are trying and not just giving up. If your coach thinks you aren't trying, she isn't going to want to reward you with putting you back in as a flyer or moving you.
At some point in your life, you'll probably have to know how to do all stunt positions, so it's good to build up your strength and diversity now. We have flyers who back and are fine at it, even though they're tiny, because they've learned how and they've worked on it. I think if you work on it you can get better.
