Kim Zolciak is doing something right.

The former "Real Housewives of Atlanta" star is already back to her pre-pregnancy body -- and showed off her size 4 physique Wednesday on Twitter.

Oh yeah, did we mention it's only been 9 days since she gave birth ... to TWINS!

Even the 35-year-old's daughter was surprised by her post-baby slim-down, echoing our own thoughts by tweeting "How is my mom a size 4?"

It's clear Kim continued to work out during her pregnancy and maintained a healthy lifestyle, meaning most of the weight dropped once those babies popped out.

Zolciak and hubby Kroy Biermann welcomed twins Kaia and Kane on November 25.

The twins are the 3rd and 4th children for Kim and Kroy, who also have sons KJ, 2, and Kash, 12 months. The blonde beauty also has daughters Brielle, 16, and Ariana, 12, from a previous relationship. 

See more post-baby slim-downs in the gallery above. 

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