Selena Gomez -- Swimsuit Upgrade ... It's All About the Fit! (PHOTO)

Selena Gomez ditched her ill-fitting pink bikini for a smoking hot black number as she partied again Thursday in the water while on vacation in Mexico.

Gomez took some flak for the previous day's photos -- haters said she looked like she put on a few pounds -- but swimsuit experts (in our newsroom) say this black one just fits her new body type WAY better than the pink one.

Selena responded to the criticism on Instagram -- borrowing a page from the Cindy Crawford "post a hot pic after a bad one comes out" playbook -- saying she's happy with her body.

Based on the pics she's posting ... she's also very happy chillin' at Casa Aramara -- Joe Francis' estate outside Puerto Vallarta.