The 12v battery is charged by the hybrid system, not the ICE, but obviously if the hybrid battery gets low the ICE will start up.
Having a bigger battery in itself won't mean it needs greater charging - only if you use that extra capacity, and in the times you need that extra capacity (e.g. three weeks on holiday) I'm sure a little, almost undetectable, drop in mpg is way less of an inconvenience than not being able to start your vehicle.