Maybe a breast expansion/Giantess growth caption?
They are sisters or cousins but the bouncy one is the formerly flat and shorter of the two who either had a second (incomplete) puberty or has been taking several experimental hormones in a closely guarded secret from everyone.
No wonder she's so bouncy! Going from flat to having a fatter chest than your busty, popular sister is quite the accomplishment, and she's not even done growing! She's been catching up on height too, she used to be almost a foot shorter, and look at her now!
Her genetics are just too good, and she's expected to grow even more after her 26th birthday where they were matching outfits, making the size difference be even more obvious... Of course the older sister wasn't so excited...being removed from the spotlight from sudden growth spurts from her youngest sister who she teased about being flat all this time...
And that size difference was only accentuated once the smaller (now bustier) sister started going into secret growth experiment labs, getting chemicals and hormones that'd further boost her natural growth, to the point that the next time they wore matching dresses, while still in the dressing room, the dress literally EXPLODED off her chest, sending her bouncing breasts, exposed, jiggling everywhere in her older sister's face. They both gasped in awe (and jealousy, in the older sister's case) about the incredible expansion they just witnessed, with tits two to three times bigger than her own head now bouncing on her chest, taking several seconds to stop, and restarting the wobbles after any tiny movement.
She was eye to eye with her older sister too...
"Will my little sister help me find a better fitting dress~? We can't go with matching clothes anymore, we're so...different now, teehee~" She shamelessly teased, groping herself and winking.
Her growth was only just beginning, yet she was already being a menace to all the friends who teased her for being flat before, giving them a taste of busty retribution~ <3