Most obvious difference is so-called "saggy" chest armor (way taller than the ROTJ version) and the armor neckline on the back is more like a tank top in Mando and squared, not round. The peak above eye lens is flatter in Mando (more so on the figure than the costume) but more curved in ROTJ, and in Mando the shoulder armor is more straight and tube like and longer vs curved and smaller in ROTJ, the bicep armor is a somewhat different shape, buckle more square, and hip armor more boxy and fatter, and the airsccop backpack has differences.
This is ROTJ on left, Mando on right:
YES NOTHING MAKES MORE SENSE than leaving the cervical and upper thoracic spine exposed on a soldier's kit! Sheer genius! Yes indeed, Force-choke whatever Imperial mastermind thought this one up.
Worst. Armor. EVER.