These are made of a stretchy polyester fabric, which is lined at the front, which makes them feel tough and durable not thin and skimpy. I bought waist 28"/70cm for my 12 year old (slim) boy. When I measure them without stretching the fabric, round the waist they are 24"/60cm, but do stretch bigger, thus giving a smooth fit.
The sizing in the label reads, GB 70cms/28" and for the other countries,
F 11/12, D 140 I 11/12 Aus boy 10 height 140cm.
They also have a drawstring which goes right round the waist so can easily be pull tighter if they are too big. Wash at 30 degrees and don't tumble dry and you're done. excellent Just what any boy needs.