Note: CHD is highly recommended over obsolete formats like CSO for Dreamcast, as it handles the audio tracks better. The World's SMALLEST Dreamcast Games!
Before we go further, a critical note. However, emulation is legal.
Some exclusive compressed sets include fan-made patches for widescreen support or VGA compatibility. Best Formats for Highly Compressed Dreamcast Games
These are 1:1, uncompressed "exclusive" rips of the original disc. They always occupy 1.1GB, even if the game data only uses a fraction of that, because the image includes "padding" (empty space used to push data to the outer edges for faster loading).