Apple's Photo Library isn't quite as proprietary as implied. The original image is stored in its original file format. There may also be a separate "processed version" that is either HEIC or JPG. These individual image files are managed by a database and appear to be a single image in the library — but they are not particularly proprietary. As stated, the operating system will transcode to JPG on export (if necessary and unless otherwise specified) for compatibility. Sometimes, for example in Apple Mail, you can control the size of the exported JPG, but I'm unaware of a way to control this from the photo picker.
JPG files are less efficiently compressed relative to HEIC. This may in itself explain the discrepancy I'm seeing — but that discrepancy feels too large.
As a workaround, my standard IOS workflow for DPRev is to export a downsized version of the image to the file system, then upload that version to the site. Unless the discussion is technical I'm disinterested in encouraging pixel peepers, so I elect to downscale the resolution — but I could just as easily provide a full-sized image that is more highly compressed. I did this for some of my earlier image uploads here — before I decided to return to sharing lower resolution images.
Try as I might, I have been unable to export an oversized image from my iPad. It doesn’t get compressed — it never goes through.
Edit to update: When I try to upload a file larger than 4 GB from the File System the site returns an error message informing me that the file is too large. I just tried to import the same image directly from my Photo Library using the photo picker and received the same error. This morning I wasn’t getting — or failed to notice — the red-boxed error message when using the Photo Picker.
Second update: The above was using my iPad Pro 12.9. I just tested using my iPhone 13 Pro. I got the error message when trying to use the File System. I got no response from the system when I used the Photo Picker — but the message composition box hasn’t become active again. (Same behavior as this morning with the other iPad.) I’m going to let it sit for a while and see if the image is eventually accepted.