The i3 REx gas "capacity" was originally software limited to just 1.7 gallons in order for the AER to be > than gas range. This allowed them to be have a different classification with more ZEV credits and additional CA carpool access.
The Clarity has none of those restriction and the tank size is not affected by any regulations/credits/incentives. Purely an engineering/design/marketing choice by Honda.
2018 Clarity Electric (Vortex Blue)