Re-reading my post, I suspect my remark could be misunderstood or seen as cryptic.
So...a slight correction/clarification:
I'm not talking simply about decreasing interior volume. That does happen, as well--occasionally, rarely, for the better, but most often to the detriment of the foot.
The real issue is that adding any kind of insert/insole into a shoe changes the insole shape; changes the relationship of the foot to the insole, and, as a result, changes the fit.
This is the same as buying a shoe that is a half size too big in the critical heel-to-ball measurement. It's what's known as the "orange peel effect."
Compounding all that, if the shoe was initially bought too large/long and the insert is being used to take up some surplus, you can end up with a shoe that is terrible for the long term health of the foot.