Imagine you made a bunch of dots on a person, all over their body, and you assigned each dot a number. Then you did the same thing to a set of armour. You put the armor on the regular sized person so all the numbered dots on both the person and the armour lined up. Then you could make a person bigger or smaller, but the dots would remain in the same relative locations, by effect shrinking and growing the armour. Sometimes, an artist would review how the armour grew and shrunk, and make small custom adjustments to make sure it looked good.

This was basically how it was done in the late 90s and early 2000s when mmorpgs were just starting to come into their own. Theres more complicated methods that have been added onto this basic method to make armour look even better and change shapes even more, but that's far beyond the scope of this sub.