I noticed the same behavior with a similar shortcut (sharing the image instead of showing it) when running it from the share sheet. When I ran it from the widget or within the app then it didn’t resize the image. Because of this I think that Shortcuts doesn’t have enough RAM available when run from the share sheet because iOS is limiting it and thus it automatically scales it down to not overshoot the limit.
There does seem to be some correlation between how you invoke the shortcut and the downsized photo. I modified my test shortcut so that I can either call it from the share sheet or just open it and select a photo. It always downsizes the photo when called from the share sheet.
Not sure if it’s a memory issue or not. With 6GB on a 13 Pro you’d think there would be enough.
In any case it would seem to be a bug. 1) it there’s not enough memory it should display an error and quit and 2) even if it’s running out of memory it seems a bit extreme to downsize an image from 2916x2916 (in my test case) all the way to 360x360.
What do you think? If it’s a bug, how does one go about filing it with Apple?