Everything depends upon whether it is a regular or hollow point bullet. The critical difference between the two is the presence of a cavity, while a regular bullet is a smooth dome-like shape. Hollowpoint bullets have a shell to permit expansion upon impact. The purpose of the hole in the hollow point bullet is to allow expansion upon impact.
Once a hollow bullet hits, the force on the front bullet forces the soft lead to buckle and expand outwards. Although regular shots also experience expansion when it hits a solid target. However, it is less likely to do so when hitting soft targets such as human flesh.
Expansion is an attribute in bullets for many reasons. It can also do substantial destruction to the mark compared to regular bullets. This damage is also because expanding bullets utilize all their energy on the target.