Most of the time resizing to 75% to make a screenshot look like …. well, a screenshot. When it is slightly reduced in size, then it kind of visually looks like an image embedded into a post because all elements in the image are smaller - it looks like something separate. Whereas if I leave it 100% size, it “blends” with the existing Discourse interface beacuse the UI elements in the image are roughly the same size as the UI elements of Discourse - and it looks ugly.
Finally, when it is a small piece of UI that I took a screenshot on a Retina screen and then embed it into a post, it looks gigantic, so I have to downsize is 50% or more.
At the end of the day, I try to make all screenshots that I insert look roughtly a bit sized down compared to the Discourse UI - then it looks very nice and is easy to read and comprehend as part of a longer text.