"So flattered by this @mirandalambert!" Spears captioned a photo of Lambert's new Women's Health cover. "My sister @jamielynnspears just showed me. You look amazing!!"
The picture shared by the "Work Bitch" singer also features a quote from Lambert's interview with the magazine.
"Like if I put on Britney, I can picture Britney's body and I'm like, 'Oh! Faster!" Lambert said while discussing her workout habits.
Lambert's figure has been making headlines ever since she stepped out looking considerably slimmer at the Country Music Association Awards last November.
"You know what, I fluctuate all the time in my weight -- it's really annoying -- but I'm going to be 30 in like four days, so I'm like, 'I need to get ahead of this thing!' Because they say when you hit 30 it doesn't get easier," she said at the time.