when you upload cover art, MusicBrainz scales down the original to make 250px and 500px versions. It does that by scaling the longer dimension to that size. However e.g. with CD front covers, booklets, and back covers+spine, they all have the same height, but different widths. If the scaling was so that the height of each is 500 px or 250 px, then the previews would have consistent DPI. But as it stands, the resolutions are often all different.
Do you think the default should be changed?