Cordell Kintner
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Its reach doesn't change. In fact, its size doesn't technically change, it's just that creatures believe it is that size of creature.
Typically, a Glabrezu in human form isn't going to be fighting. Rather, they will be trying to gain the trust of the party in an attempt to lure them into a Miracle per their "Twisted Desires" ability, or so they can stab the party in the back at a later time. If anyone somehow sees through the disguise, they would see the Glabrezu's true form, and thus their true size.