It turns out I was correct. After shaking the soda and pop rocks multiple times, it still could not measure up to the size of the vinegar and baking soda balloon.
Some things that went wrong or not as planned were the pop rocks, the size of the balloons, the flexibility of the balloons, and the different types of soda each group used. The pop rocks' instructions were to "fill the balloon with one package of pop rocks". The balloons we used were entirely too small to fit an entire package of pop rocks, so that probably skewed the results. The balloons were not flexible at all, the would stretch, but were even hard for me to blow up! My group used Original Coca Cola soda, while others used everything from Coke Zero to Sprite. That could have differed the results between groups.
Our groups balloons looked somewhat like this:
Pop Rocks and Soda: Long with a round bulb at the bottom (after being shaken)
Vinegar and Baking Soda: Filled up until the top, were it was almost like a small tube.