As a very slim young adult (28 waist with muscular thighs), the larges fit me much better than the Hanes of the same size even though I prefer Hanes for undershirts.  I have worn briefs ever since I was a kid, and have tried many different brands since.  Advertised waist size being the same, the cut of the Boys size L Fruit of the Loom is roomier around the crotch area and probably better suited for most older boys and slim men than the Hanes equivalent.