It's not the size of the battery - it's the balance of use and reserve capacity to store extra charge. As Mr. Natural stated, a bigger battery would require more generation to fill it. The current battery is a good balance. You'd never want to purposely fill the battery as the energy used to do that is more than what you'd use to directly drive the wheels due to storage inefficiency/overhead (aside from hill descents). That said, the absolute sweetest spot on the power gauge is the white line between green and blue. If I have a choice between banking energy or being net neutral (without breaking the speed limit), I'll take net neutral every time.
With respect to reaching full charge, it's not something to purposely shoot for. The battery should be considered a reserve to dump excess energy for later use. The goal is to drive conservatively and anticipate in order to grab excess from hills and coasting. Braking costs energy as you spent it to get up to speed (aside from hills), so bleeding it off through braking is kind of like a foreign currency exchange you pay a fee to do so.
I larger battery would only make sense if the generator production was able to be increased.
This is a hybrid, not a pure electric vehicle. It doesn't require a monster battery (along with the weight that goes with it).
