It's a good problem to have, but as long as the heavier girl is not butt-ugly and she is at least an HB6, I wouldn't worry about it too much.
Girlfriends lie to each other all the time, so even if the fatty does tell the hot one what happened, the hot one may think she is embellishing the story. A bit of healthy jealousy is good anyway, because it will increase your pre-selection / social proof and show that you're experienced. Remember that women don't necessarily want monogamy from a guy they are thinking about fucking. It's better that she thinks of you as a wild partying player than an infatuated loser with one-itis. Guys look for loyalty in women above all else, whereas girls look for confidence in guys (loyalty is much less important, hence why they stay with their "badboys" even when they get cheated on.
Just make sure that you're ambiguous about what happened and just say something like "yeah that night was pretty wild" and imply you were just having fun partying, nothing serious. She's not your girlfriend, so it's none of her business what wild fun you got up to after she left. Never make excuses or apologise to a woman, especially one you haven't f-closed, or it makes you look weak. If you meet them both again, be nice to the fatty, but be clear that it is the hotter one you're interested in. If they both seem DTF, then try and get a threesome vibe going
