This suggested edit has “added 3408 characters in body”, yet the diff only shows 11 added characters. Why?
As a <2k user across the network, I think I can shed some light here. When a user goes to re-edit a post on which he has a pending suggested edit, the software automatically fills in the edit summary from the most recent version of the suggested edit. So assuming that the behavior hasn't changed since 2011, a likely explanation is that the user in question did initially propose an edit at 10:04 that added some 3000+ characters, and then, over the subsequent 2-hour period before rejection, modified the suggestion at least once until it became the one-line change we see preserved in the linked review. If the user never bothered to touch the edit summary box throughout the whole process, the automatically-generated summary from the initial proposal would remain – completely untouched – on all subsequent versions of the suggestion despite no longer being an accurate reflection of the content.
There are just two cases when this would happen
It's probably that first case, but I have no idea why would OP do it
Could very well be simply a huge amount of whitespace added.
They don't show up in the diff, but the characters are counted towards that number.
(Random edit to a sandbox post here - shows 186 chars added to body that are not visible in the diff.)