The reason there isn't much volume of air in the skimmer is that I have the air intake severely restricted. Restricting the air intake decreases the air flow but also decreases the bubble size dramatically. I've found this configuration to work the best for this particular skimmer. Restricting the air intake in this way is also what allows me to run a powerful pump on an under-tank skimmer. If I unrestrict the air intake, the skimmer will definitely overflow out the collection cup.