If your AC system is too small for your home it’s going to have to work harder in order to cool the space. When a unit is new it might be able to keep up with the increased work load but over time it will wear down on the AC unit and cause issues and break down.
A dirty air filter will reduce the amount of air flow reducing the amount of cool air the AC unit can produce. Always keep an extra air filter on hand and remember to change them regularly.
All AC units need regular maintenance in order to perform smoothly. Maintenance should be done in the spring and the fall.