Think of 'duty' as the job a thing has been handed. Two bags hang side by side in a shop: same shape, same size, but one has been given the heavy job and is built for it — thicker plastic, wider seams, a base that will not let go. Heavy-duty always points at that second one. It is not a comparison with the whole world, only with the ordinary version of the same thing, and the word never really tells you the weight; it tells you how hard the work is going to be.
Two branches
為粗重工作而做的東西
這兩個意思直接用那個畫面，只是被交代工作的東西不同：一個是材料和日用品，一個是機器和車輛。Wiktionary 甚至把兩者寫成同一條（hard-wearing），可見中間的界線很淡。
adjective · B2
Made of strong, thick material that can take rough treatment for a long time without tearing, splitting, breaking or wearing out.
厚實耐用的
被交代粗重工作的，換成了人
這一家把同一個形容詞從東西移到活動上，被操的對象從袋子變成了人。這是概念上的延伸，方便理解，不是在說哪一個先出現。
adjective · C1
Very serious, very intense, or asking a lot of you — used about things like work, study, talks, cleaning or training, rather than about objects.
很硬的／很猛的