What the prop-builders at Lucasfilm originally intended was a vest (and abdomen-plate) that finished where the belt began. They achieved this very well in ESB, but the ROTJ version and the Pre-Production has a rather casual outfit, considering the position of the armor. Rumors in fandom say that the Fettsuit was intended for a smaller actor, so the armor exposes Fetts stomach (weak point, no bounty hunter would let this happen). The original intention can be seen in the Comic-book "Twin Engines of Destruction", where Jodo's and Boba's stomach is well protected, and if you are true to yourself - This looks much cooler than the too short vest, doesn't it? So the exposed stomach could be a glitch, while Lucasfilm corrected this in Episode 2 and made Jango wear the armor as originally intended. Therefore his armor is obviously bigger, I suppose. However, many drawings of Fett still show the short vest, because it appeared in the film like that.
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In the comic, it's possible to have the armor always in right position, while in real life it while inevitably change position when moving.