the word smaller is used to describe a comparative degree of size, in which the object being described has less dimensions, mass, or capacity. It is used to demonstrate that an object is lesser in one - or more - of those qualities when compared to another object.
1. This sweater is smaller than the other one.
2. His house is much smaller than theirs.
3. She picked the smaller cupcake from the tray.
smaller is not often confused with words like 'little' and 'tiny', which are more suitable for describing something that is considered small in general, rather than by comparison. It is important to know that smaller is always used for comparison and not for general description.