I reckon many anime group encoders these days don't have any settings for a targeted file size but rather some mathematical level of quality that would depend on how much changes from frame to frame throughout the episode. In an episode with lots of action and effects, that number would be high, resulting in a big file size. Similarly a static episode with only talking heads would give a small file size. Thus all episodes in a season would look the same, quality wise. Otherwise calmer episodes would have better video than the ones where stuff happens.

All in all, I would also accept smaller file sizes. I'm not a quality freak anyway. It's kind of annoying that when the prices of bigger HDs slowly get more humane, in fact relatively speaking they aren't anymore any bigger because the stuff you save on them has got larger as well.