General

Me and my girlfriend are starting to do mukbangs soon and she doesn't know i have this fetish, she just want to try doing mukbangs with me as she likes eating. But i would like to gain weight with her so we can both become rounder since we are both quite skinny. How should i tell her that i want to become fat with her without scaring her too much?

Just let it happen that way you don't have to tell her anything right now.
You young people are always making things harder than they need to be.LOL

Yeah true true ahah, i just don't want to scare her too much with this if she is scared of becoming too fat

If you feel so compelled to tell her thats up to you but if she has a fit and hates you for saying it then that shows you should have kept your mouth shut.
Just keep quiet and tell her how great she looks as she puts on a few pounds.
Never make it about her size make it all about her.