The title of Foxy's most recent video echoes what I have been hypothesizing for quite some time; "MY FILLS GIVE ME BOOBIE GREED"
With most girls, having the ability to make their implants bigger is a real desire, however, some, like Foxy, feel a need to keep making them bigger. IMHO, expanders satisfy the desire and feed the need, their boobie greed, that is only quenched when their boobs reach a size where any more would become more burden than they seek. Few want boobs so enormous that they impede normal movements, changing everything about daily life, so they stop expanding. Foxy wants her boobs so massive that she could no longer do half the things a normal girl could do, but life would be far from laid back because everything would be more difficult and take more time with your boobs always in the way.