The 18.5 gallon limit is by default, not design.
The original target was 20 gallons reduced from 22 for the new smaller V6 turbo motors. But the leeg mandated cell mfg produced cells that ranged from over 22 to down to 18.5 gallons. There being no way to expand the capacity of the smallest cells, the smallest became the defacto new rule. Thus the teams that received the larger cells have to run with a brick or two (or more) in their tanks to meet the new limit. Changing the name on the building sign at 16th St. and Georgetown Rd. has sure made everything all top flight and professional and shit.