) posted a of herself inside the fitness facility.
She walked closer to the camera to give viewers a better before-and-after look.
The exercise fan donned knee-length tight-fitting shorts and a bright blue sports bra.
Her butt was framed but not as plump as it could be in the bottoms.
"POV: you find out how fitness influencers wear their shorts," her caption read.
Millie grabbed the material on each leg and rolled it up to the top of her thighs.
"This is how," she said.
She turned around to show viewers the difference in how her butt looked.
With the shorts rolled, Millie's butt was propped up.
Over 1,000 stunned viewers commented on Millie's post to convey their shock at the easy hack.
"Bro went from caked up to bakery owner," a fan exclaimed.
Another YouTube user agreed: "Girl ain't got cake, not the bakery, but the whole d**n Pillsbury dough boy company."
"Dude, you just blew my mind," an excited follower admitted.
A skeptic thought Millie's hack would only work if you already had a big behind.
They wrote: "Fun fact this still only works If you have a a**."
An appreciative viewer thanked her. "Ooh this is useful thank yoooouuuuu," she commented.