e621 runs on one rule (as well as many others obviously): we are trying to get the best version of an image. and so that implies if an image is of the same res but has a much larger file size then it is CLEARLY the superior image and therefore should be the one that gets to stay.
This has happened to myself numerous times. I post something and then later someone else posts the same image with the same res, but with just a slightly larger file size. and mine gets deleted. so how come when I do that same thing, people start telling me that MINE should get deleted???
Updated by Ratte