There might be several things spawned from the initial upload and kept around. If i click on this post, initially im shown a square thumbnail, a ~20kb .webp file. If i click on the image, it shows me another .webp which is a bigger file and it doesnt fully show on the screen.
Clicking a third time opens up a .jpg for me, as best as i can tell its the original upload.
Maybe whats served depends on the view or app each person is using… It might be further complicated if the server knows whether the person is viewing on a mobile device and try and serve lower resolution. . But i try and upload at atleast 1920x1080.