Why faster space railgun rounds might still need mass

Could a space railgun use a smaller projectile because the shot travels so fast?

Yes to a point, but uncertain accuracy and the need for damaging effect still favor a projectile with substantial mass.

A follow-up asks whether a space railgun projectile could be smaller because it travels so quickly. Speed permits some reduction in size, but accuracy creates a countervailing problem: a weapon crossing great distances must still produce useful damage when it hits. The design choice therefore balances velocity against hit probability and terminal effect rather than minimizing caliber alone.