As others have noted, blocking edits that are too large can prevent legitimate edits from getting through. Example
The real issue is the one described at Improve filtering of spam suggested edits. Spambots have recently learned to make suggested edits to Stack Exchange posts. There's still some work for the SE team to do to detect and reliably block spam suggested edits. For now, human reviewers will handle this issue, and this is why we have the review queues in the fist place.