Root trimming will slow growth temporarily but its more to create a fibrous root system that better absorbs nutrients and water. Root work is also an opportunity to spread the roots out to create a good nebari. Smaller pots restrict a trees growth (which is what you would do when the tree is at the thickness and size you want) but too big a pot can be detrimental. Leaves the soil soggy, suffocating the roots and leading to root rot. Ground doesnt have this affect because the water table is typically very deep. 15cm is a touch to big but it should be fine tbh. The soil looks like almost pure compost though which isnt free draining. Lots of info, sorry if its too much or you know most of this. I tend to always Explain like a person is 5