Based on this fanfic https://www.fimfiction.net/story/467927/giantess-and-devious
 
Tired of suppressing her primal desires Chrysalis approached Luna for a way to address the problem, prompting Luna to teach her a special kind of spell for lucid dreaming. Using this Chrysalis dreams up a scenario where she keeps growing bigger and bigger, crushing Manehatten and Ponyville before gobbling up the Princesses of Equestria. Unfortunately the spell meant that every specific pony she chooses to involve has this dream in turn, to Luna’s deep regret.