It's arbitrary in the same way measuring the time between photon absorption/emission in a cesium atom is arbitrary or the rotation of our planet is arbitrary.

Picking the smallest is arbitrary just like picking a larger interval.

In the cesium clock case, you count 9192631770 because it's close to 1 second we already are familiar with and arbitrarily say 9192631770 transitions is defined as 1 second.

For example Planck time is defined as 5.391247(60)×10−44 seconds. But what is that second? It's the arbitrary 9192631770 cesium transitions we picked because it's close to the second that come from Earth's spin.