There are dry bags and then there are dry bags --- I don't use the heavy rubberized type except for kayaking trips (i.e., when the weight isn't on my back ...). The lighter ones aren't all that bad, and inside an overall pack liner are pretty bullet proof, so even in deep stream crossings, etc I've never had a problem (plus I live in the pacific NW, a notably damp climate overall).

The largest dry bag I have for a sleeping bag is an 18 liter Granite Gear bag, a large fit for for my 10F rated down bag. This dry bag weighs 77g (2.7 oz). More typically I'll use a dry bag that's under 2 oz, and for my summer bag I sprang for a cuben dry bag --- don't recall exact weight, perhaps 1 oz tops. I.e., not that heavy.

Also not as truly waterproof, perhaps; these sort of lighter dry bags will wear over time and become at best "less water proof", but again, inside a yard waste pack liner, it's pretty good protection overall.

As with so many things in the world of backpacking, there's no single "right way" to do it!