Your bay tree’s roots will eventually fill up the container to capacity. Generally, you need to re-pot your bay tree every two to three years.
Instead of constantly expanding the pot, pull out a third of the roots and re-pot a plant into the same container with new soil encircling the now-smaller rootball.
Scrape away all of the compost on top and add new compost in its place. In the spring, when the bays are about to begin growing once more, finish this work.