which is the best way to induce the thickest trunks? Either let a tiny tree
grow up as tall as it can, or cut it down at the top when its about 30 cm
( 1 foot ) or similar high ? thanks.

Hi JW,
Firstly, it depends on the type of tree. Some trees especially if they are grown from cuttings almost never grow thick trunks.
Growing up as tall as possible then cutting it down will certainly help the trunk to thicken but then you have the problem of what to do from where the cut is. Usual is to cut just above a branch and then train this up as the new leader/apex, but this can cause a bend in the trunk where the branch grows from. If the new leader is then allowed to grow for a season or two and then this is cut down and a new leader selected you end up with a nice taper on the trunk. If you do the initial cut say at an angle from right to left, then do the next cut at an angle from left to right - this will even up the trunk somewhat and can be done as many times as you want - each time cutting from a different side of the trunk.
Another way is to let some 'sacrificial' branches grow totally wild for a season or two - or for as long as you can. This will thicken the trunk considerably below the branch/branches. When the trunk is as thick as you want it, these sacrificial branches are then removed.
Another way if the pot is large enough or is a training pot ie: black plastic or styrene box - and especially with figs is to cut all the downward pointing roots off, and then put a 'plate' under the tree, spread all the sideways pointing roots evenly over it, wire or tie the roots in place then bury it. The plate can be anything that won't rot or break up very easy. If the trunk is only relatively thin an old CD is ideal, or a piece of fibro/cement sheet. I wouldn't use treated timber of any sort - they contain pretty toxic chemicals and can do considerable damage to the tree if it absorbs any of them. Hope this is of some help to you
TinanaTaffy