I have to suppose that motivations may differ when you go past a subjective size where your breast hinder normal life. Both Foxy and Chelsea love the attention, however, Foxy has intentionally made her boobs huge while Chelsea just allowed it to happen. I would like to think that you have to love having huge boobs on a personal level to have them to start, but Chelsea apparently had negative feelings about the way they adversely effected her life. We all do what we have to to earn a living and thus Chelsea used her gift to her advantage. Which brings me to Foxy.
I doubt that Foxy had any intention of becoming a huge boob sensation nor think of it as a occupation when she started; it was just a desire to transform herself into a image she desired. That image had large fake boobs, but certainly not huge, however, IMHO, Foxy just got caught up in her breast expansion, wanting more. If that surgeon had not offered her expanders for the same price as the 2000cc implants she initially intended, where would Foxy be today?
Today, Foxy makes a living off her looks and she loves the attention her huge boobs create. I'm happy that she getting the attention she wanted and even more so that she seems to love having such huge boobs, after all, when you have made your boobs this big, it's not like there are many jobs you can physically do and life in general is now filled with challenges like never before. But when you actually love having such huge boobs, you tend to embrace those challenges as affirmation of how big you have made your boobs. The girl in love with her boobs will then want to make them even bigger, fully aware that the challenges will just multiply; Foxy has that desire. Bigger boobs will not necessarily proportionally generate as much additional attention as they will alter everything in life, but when the desire is deeper, the satisfaction in becoming somewhat overwhelmed by your own breast size is reward in itself IMHO.