Could big teeth be made smaller?

2 ways: 1) if teeth are out of position they may look more prominent. Straightening them may be all that's needed. 2) if teeth actually too wide or too long they can be restored to look smaller. Sometimes both are required.

Yes and no: Depends upon the reason for making smaller, and how many teeth involved. You do not want to sacrifice function and balance by having any radical treatment on your teeth. I would get more than one opinion. The appearance of making teeth smaller can vary from lightly grinding the edges of the teeth or more radical treatment- veneers, crowns, etc.

Tooth size: Big teeth can be made smaller and small teeth can be made bigger. The question becomes why are we changing the size and is it a realistically achievable goal from the standpoint of function and comfort and homecare maintenance.