Most store bought kitchen drawers are metal sides which incorporate the runner. I have just made 12 dovetailed drawers for my bench & made them from 20mm thick stock. For furniture applications i usually make my drawer sides 12mm to 15mm thick & the fronts about 20mm thick.
Hope this helps,
Just guessing that he used the 20mm sides on his last project because it was for his bench; likely some heavy stuff in it if it is like most of our shop drawers.
That's right, the drawers in my bench will be mainly for tools & i wanted to beef them up a bit. If i am using full extension runners i use 12mm stock all round & apply an extra drawer front about 20mm thick to cover the runners. If no runners are used i sometimes use 12mm on 2 sides & back & 20mm on front with half blind dovetails on the front.