As a guy in his 20s, if you're both attractive, but you're not as heavy as she is, that is likely the reason. I honestly don't even know what people our age call thick because I never integrated with you all. To me thick just means curvy, fat, overweight. So unfortunately heavier will always be less attractive.

On a different note, personality. You said she loves male attention. How does she express that. That could also be a turn off if she begins to act like the wild obnoxious girl who you can tell wants attention.

Compare the guys who give each you attention. Compare their personalities, presentations of themselves, how they approach and talk to you both, etc.. Maybe you have an allure that attracts guys seeking that charm which she lacks.