Please don't get me wrong, this is not about being somehow... shy when editing - quite opposite, I firmly believe that one should feel completely free to edit and the more, the better.
Neither this is about amount / size of content to add - I for one never feel anyhow obliged to correlate size of my edits with that of the original post in any way.
Nor this is about limiting self in making justified cuts to original text - I cut without mercy when I can justify (preferably, in the edit summary) my cut to anyone, including original author.