The smaller cup sizes they have available means you are less likely to load up on fro-yo and toppings compared to other places. It is kind of like that old diet tip, use a smaller plate and you are less likely to overeat with bigger portions. It made my bill nearly 50% less than it's competitors and I was just as satisfied, and with the extra money left in my pocket, more likely to come come back.
Bonus: One of the people in our group was craving a cupcake instead of frozen yogurt. They had those too and it was supposedly good so everyone was happy.