Hmmmm....one thing about bigger battery packs is that they weigh more. Yes, larger packs are better if you have high current needs, but a 600 mAH pack in almost all sport planes is plenty for about 6 10-minute flights. If you want to do more, then just take a field charger with you. You can add quite a bit of charge to a model in 5-10 minutes between flights. That will allow you to fly all day, and you'll not have the weight penalty of the large battery pack.

Just another option.