EoSD bubbles aren't the issue - they are still sensible despite the bigger hitboxes. But my curiosity is seriously, seriously compelling me to learn what the exact hitbox is for the metal-fatigue bullets. I know they seem as big as the sprite, but I've seen a couple replays that suggest the hitbox does not, in fact, cover the whole sprite. I even tested it tonight on Young Demon Lord (if that makes any difference) and, judging from my movements, managed to slip into the top-left corner of one (the same place I've seen others do it). Reproducing it has been a problem though, since I don't have any sort of "action replay" type of equipment, and timing a "print screen" button press while simultaneously using my pinky to control focusing hasn't proven very productive. If anyone else with more expertise can provide some info, I'd really appreciate it. It would certainly help if we can settle this issue once and for all.
Just because it is technically possible to do something like this doesn't mean that you should try unless it is just for showing off.
These are two totally different engines you're comparing. I'm not sure the analogy works. Never mind that slipping through such small spaces on Medoteko is sometimes necessary.