The reason why protein skimmers produce so many bubbles is because DOC’s adhere to the surface of bubbles. Therefore, a higher number of bubbles means an increase in the surface area of bubbles that the DOC’s can adhere to, resulting in a higher efficiency of DOC removal.
The DOC’s/proteins that are adhered to the bubble, stabilizes it very much like a soap bubble- they take a long time to pop. This is why you get a ‘foam’ collecting in your collection cup. This is all the DOC’s accumulating, ready to be removed.