I believe that originally the Funk was designed to be larger, but in the final cut the nose was shortened to make fitting into a garage and parallel parking spaces easier. This also provided additional pedestrian safety and aerodynamics. The smaller frunk was offset by the the additional benefits. The new frunk also works as a rain/sun protected seating area.
Tesla works hard to make their frunks as large and useful as possible, but other issues are considered as well.
With the huge protected rear bed being so large, the frunk is less critical.
End result is that the CyberTruck is shorter overall than competitive full two row seating plus full sized beds. Fits into garages where others need to be parked outside.