as noted in my previous post, i think it could be on-the-fly conversion. you might have, say, a PNG or TIFF file stored on your machine, and the Photos app might be converting it to JPG on the fly when you drag it to the upload page in the browser (or something like that). (i don’t think a JPG file of your reported pixel size could get anywhere near 40MB.)
here’s a thread where we talked about how the Photos app will convert HEIC to JPG when files are dragged over to the browser: https://forum.inaturalist.org/t/a-different-heic-question/33028/24.