Two skin spas in one day! And we're guessing it's not just Botox and lasering ...

Kim started her Tuesday at Epione Cosmetic Spa where Dr. Ourian (Kim gives a literal shout-out to him in our video) worked on the ever-shrinking star. There's no doubt Kim's famous booty and thighs have gotten progressively smaller over the past few months -- basically since she was robbed in Paris last October and subsequently went into "hiding."

It's our best guess that Kimmy K is slowly reversing a Brazilian butt lift procedure. It's been reported the Kardashian sisters all got it done, but Mrs. West is the first to change her mind about having such a massive derriere. It's possible to go back on the decision, simply by liposuctioning the fat layer, according to top docs. If the fat is pressed deeply into the muscle layer, it then becomes irreversible. A little nip/tuck of the stretched out skin and voila... a more natural backside!

A source tells X17:
    "There are a lot of reasons Kim wants a smaller butt. She wants to stand out from her sisters, she wants to fit into designer sample sizes, and she doesn't think her huge butt will look good once she really starts aging in a couple years."

Khloe, fresh of her Caribbean vacay with her love Tristan Thompson, went along for the sisters' day out, as did Kourtney (finally, a few hours away from the kids!). The trio hit up Epione to start and then hoofed it across town to Santa Monica for a visit to Nurse Jamie's skin spa. Jamie has worked with Khloe on Revenge Body so the ladies have become big believers in her treatments and skin products.

All the girls were dressed in black leggings and sweatshirts, Kim and Kourtney also carrying crocodile Hermes bags. It was a casual day out with a mission -- better skin! Seriously, we don't know how these ladies' skin could get any better ...