Usually you will have a base-mesh to model around, something generic and tubular to keep things simple, the clothing itself can be as crazy as you want, but you will usually want to have (or at least need) something basic to adhere to, for later down the pipe-line as far as bodies go.
From there, by using whatever tools you have (such as brushing, pushing modifiers, morphs if base uses, etc) you push and nudge (or morph) the clothing to fit the new body and fix any issues it might have (such as poking holes, etc).
Please note most games nowdays use Morphs, and usually as OGLU said, scale bones, so your clothing will grow (pun intended) on you as you change sliders.
For games that have unique bodies and don't use morphs and scaling, it's doing it manually (if you had a good starting point, pushing around the model to fit the new body should be easy, outside of re-rigging).
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or scale the chars afterwards...
From there, by using whatever tools you have (such as brushing, pushing modifiers, morphs if base uses, etc) you push and nudge (or morph) the clothing to fit the new body and fix any issues it might have (such as poking holes, etc).
Please note most games nowdays use Morphs, and usually as OGLU said, scale bones, so your clothing will grow (pun intended) on you as you change sliders.
For games that have unique bodies and don't use morphs and scaling, it's doing it manually (if you had a good starting point, pushing around the model to fit the new body should be easy, outside of re-rigging).