Having a properly sized pot is essential to have a healthy plant. You should know that when plants grow, in general, the roots grow as well. If the pot is too big, two things might happen. The first scenario is that the roots might grow very big in comparison with the plant because they will have enough space to do that and they will consume the nutrients for themselves, not for the plant.
The second scenario is that the roots may not be big enough to get access to light and so they will not be able to provide the leaves and the flowers with the nutrients they need to survive.
If the pot is too small, however, there is the possibility that the plant will stop growing because there will be no place for bigger roots and the small roots won’t be able to absorb enough nutrients for the plant to grow bigger.