Sasha's growth stage increases every 2000 gold spent at the Bunny Store
Orchid's growth stage increases every 5000 gold you deposit at the Bank (you need to start her quest to start a bank account though)
Maris' growth stage increases with feeding her fish. You need to finish all of her quests to unlock the feeding minigame. The rarer the fish, the more growth score it gives Maris (on the minigame's screen, they are ordered from least to most score given, from top to bottom)
Hilda's growth stage depends on, I think, completing her quests, and then reaching certain smithing skill level thresholds. I think she reaches max growth somewhere around lvl 20 smithing? I can't remember what it was precisely.
