The thick heavy duty bag is always have big size, with single flat bottom. Generally, it is used to contain high volumn and heavy goods, requires high strength. So it is usually made of LDPE, film thickness is more than 20micron, like construction garbage bag, garment fabric packing bag.
However, some big size bag is made of HDPE, because it not requires high strength, but just used as a cover or protecting film. Such as bags for furniture dust protection, hanger bag, sanitation garbage bag.