Generally more electric power the bigger the car is going to be. For the power they need anyway.
While a larger battery may need more space, a smaller fuel tank frees up space on the flip side, if the engine is ICE is doing less work might be able to save space on things like cooling, exhausts etc too, and most importantly don’t forget this isn’t like an electric road car where you need a battery large enough to last the distance your traveling, the battery is charged up during the race’by alternators, breaking etc so only needs to hold a laps worth of charge at any one time, not 50-70 laps.