Think you could do one with one or all of the sirens gulping down water and have it go straight to their tits, ending up the size of bean bags? If not the sirens and water, could also work with Rarity and milk or Applejack and cider.

Rainbow Dash breast expansion?

How about you have one of the girls do something that causes them to grow and leaves them as a giantess with huge breasts, a huge ass,, huge muscles and a big futa penis. Basically, whoever it is becomes an all-powerful Goddess. (But their personality doesn't change) Or if you want, you can do multiple girls. These are the ideas for who I think should experience this growth:

Fluttershy
Pinkie Pie
Diamond Tiara
Sonata Dusk
Derpy Hooves
Sweetie Belle
Maud Pie
The Great and Powerful Trixie
Lily Longsocks (Y'know, that cute filly with the super strength)
Marble Pie

Definitely needs more chapters than 2

I expected more