If the "big cup" you're referring to is the refillable mug you're issued at check-in while on the DDP, it's not limited just to soda -- you can use it for unlimited [self-serve] refills of any hot or cold beverage available in your resort's food court during your stay -- coffee, hot or cold tea, hot chocolate, juice, milk, lemonade, fruit punch, etc.
If your concern is that every time you order a DDP meal, you'll be issued a huge fountain drink cup and will be limited to fountain drinks, don't worry -- you'll have more choices than just fountain drinks at most WDW restaurants (both counter service and table service), so you don't have to waste your beverage entitlement! Check the menu (adult and kid sections) for bottled water, juice, milk, Smartwater, coffee, tea, etc. Also, many places that offer milkshakes will allow you to order a milkshake as your "beverage" on the DDP. If you're not sure what other beverage options you have at a given restaurant, it never hurts to ask.
DH likes his Diet Coke, so when we were there, he and I would split his beverage at each counter service lunch(the fountain drink cups are HUGE - we'd just ask for an extra one to divide the drink) and I'd order the biggest bottle of water they had as my beverage, so we'd have one to carry around with us for the rest of the afternoon. We didn't have to bring (or buy) any additional water. (If you're traveling to Florida in the summer, however, YMMV!)
