Technicalissues

I'm struggling with a similar situation, I (24M) am in a new relationship (22F) and finding it tough to bring up. She is thick but definitely not big by most people's standards and is constantly exercising. Our only interaction around my preferences was when she went searching for a page I follow on Instagram and found a high number of plus sized models that I follow. She confronted me about it and was upset that she doesn't look like them and why would I be with her considering that she doesn't. Any advice on how to bring up my preferences to someone who absolutely has no knowledge of this world existing?