Kourtney Kardashian Defends Post-Baby Body From Troll on 45th Birthday

Kourtney Kardashian is again defending herself against people who feel she doesn't look as trim as her sisters -- saying she loves her post-baby body ... on a very special day, no less.

For Kourtney's 45th birthday Thursday, Kim K posted a pic of herself, Khloe and Kourt on their recent beach excursion ... in which they all wore little black bikinis. One fan suggested Kourtney wasn't going to like the pic, 'cause she wasn't as snatched at the other 2.

Kourtney responded to that comment, shutting down the negative talk with a self-love message ... making it clear she loves her body at any size.

The OG commenter wrote, "Now you know she's not going to like this photo... lol it's her bday Kim.. lol."

Kourtney replied ... "I LOVE this photo! It is me and my sisters having the best time on a trip with our kids...and the memories to last forever! And I LOVE this body that gave me my 3 big babies and my little baby boy."

Kourt doubled down on her support of Kim's birthday post, taking to the comments again to declare her love for her younger sister.

She added ... "I love you sooooo and so grateful for all of the incredible memories we have together… when I think it, it’s like a movie plays in my head of all of them! So blessed!"

This isn't the first time Kourtney has had to defend her post-baby body in recent months. In fact, it was just last week, KK shared her own bikini pics from the family's getaway to Turks and Caicos ... and penned a letter slamming pressure to "bounce back."

Kourtney -- who welcomed her newest son Rocky with Travis Barker in November -- encouraged her followers to be kinder to themselves as postpartum bodies are still adjusting months after birth.