i'm pretty sure it has something to do with the "no large intestine" thing
It has nothing to do with the “no large intestine” thing. I have had no colon for 26 years and I consistently struggle to avoid weight gain. Same as before I had surgery. And despite being very sick with UC.
There usually isn’t an inocuous reason for losing weight. If your appetite is good, and you are eating a proper and balanced diet, it shouldn’t be happening. Metabolism, diet, and undetected or untreated illness could all be factors.
Your GI should have ordered full blood labs to check for anything amiss. If I were you I would have suggested it.