Right before the show starts. Starlight and Trixie has to do some rehearsals. So Starlight will be volunteering and helping out Trixie's magic show. She is the role of being that bunny to be in the hat and getting pull out later. During the rehearsal, Trixie had a funny thought as Starlight is getting stuck in the hat. The hat is big enough for her to get in and stuff but Trixie decided to shrink it because why? It's amusing to see her getting into the hat because of her butt as you can see already.

That also explains why Starlight isn't amused in the last picture: