Yeah basically that I saw them as A.M.A.B. Due to both combo of them being a pranksters and being the weird kid, and more often than not artist i've seen will give Kris a lengthy build.
But Kris being A.F.A.B is fine as well. i have not seen it really done with them but, when it comes to the O.G's Frisk and Chara. I have seen them depicted either or.
And again youre not alone
So not alone, in fact, you're the least alone "accidental" headcanon in all of UTDR
In general I dont think people think about Kris's AGAB all that often, it's only really important for the most fundamental levels of character analysis, to piece together how they were socialized as a child and how that can affect them now
As such Male as Default takes effect, and people just assume Kris is Male and work with that assumption, and as such you see Kris with the broader shoulders, the Male Q-Angle, No Chest, stuff like that
Of course I disagree, my strongest argument being how Kris constantly Masculinizes, and very rarely goes out of their way to be feminine, most obviously made clear by their two chosen outfits, in the Light World they wear a sweater that can cover up and and all of the traits that could reveal their sex (so if they have a chest or how their shoulders sit, stuff like that) and in the Dark World they have an outfit that should reflect some aspects of themself, and I find it rather notable that the only pieces of armor they wear, other than the gloves and boots, are a breastplate and shoulderguards
It reads to me like someone who wants to protect and hide these aspects of themself, like how Susie's Dark World outfit goes out of the way to keep her tail hidden, if there's a part of your body that you're self-conscious about we already have precedence that your Dark World outfit reflects that
Still, that's just my thoughts about it, ultimately it's headcanon but I like to explore it a bit more seriously for fun
Thanks for the ask again