She's very slightly curvier, but the whole episode has a unique feel that makes all of them actually seem teenaged for once.
Star's less spastic than usual, there's barely any magic involved, the focus of the episode is dialogue and relationship-building rather than action or storytelling, the setting is mundane but unfamiliar outside of Marco's house, and Star, Marco, and Jackie are basically the only characters shown the whole episode, so you don't compare them to others and can't even reference them in context with the setting once they leave Marco's house, which is most of the episode.
It's all very well put together. It's the most character-focused episode yet, so they make sure your only anchor to the story is in the characters by removing all the other familiar elements.
And to enforce that the episode is specifically about Star's feelings, they even sideline Marco for most of the episode, so you're inclined to focus on her over Jackie, who is less familiar. Even when they get to the concert, Jackie isn't even in-frame for most of the scenes, and they prime the kiss with about half a minute of solo Star screentime. It's really solid presentation.