Medium Systems Vehicles (MSVs) are similar to GSVs but smaller. As the size of a typical GSVs tended to increase over time in the Culture's evolution, some ships which were previously designated as GSVs were "demoted" to MSVs. But a demoted ship with a good track record typically retains its influence (for example the Not Invented Here in Excession).
Some of the known classes of MSV are:
Desert Class - Described as having inaugurated the "Very Large Fast Self-Sufficient Ship concept" some 2,000 years before the time of most Culture novels, later recategorised as Medium Systems Vehicle. Described as "three-and-a-bit" kilometer long ship with a material hull, rather than the later design of a set of decks inside nested, ellipsoid force fields.
Steppe Class - Described as "relatively small".