My subscription ran out recently, but this was an issue even when I was current. It seems almost as if a majority of the covers in my CB are tiny (no bigger than the thumbnail.) Covers for very well-known, mainstream comics, that even if CB did only have a tiny image at one time, would have been replaced many times over by users if not HC themselves. Take a look at Amazing Spider-Man, for example. Very few covers are not tiny.
Some covers display large but are such poor resolution that they are probably meant to be tiny as well.
I feel like this is an issue that started in the last year or so, though my sense of time is not great.