That's a good point, though we may be dealing with small amounts regardless. For instance, I read that for one study of tobacco lung cancer, a "moderate" smoker starts at 10 cigarettes a day. (woah). So let's say someone smokes 10 cigarettes a day, every day, and let's say a single cigarette takes 10 moderate drags (I'm getting this by googling and using the first reddit thread I find, I don't smoke).

Ok, so 103010 = 3000 drags a month for a light/moderate smoker.

Now, imagine a casual (2x a month) cannabis user who actually smokes most of a smallish preroll, 6 drags (though they may be deeper). So we're looking 12 drags a month vs 3000.

I have no idea if the extra 8-10 drags from weaker cannabis make a difference vs 1-2 on something stronger, but we're talking about an extremely low dose, either way, compared to any kind of cigarette smoking, maybe low enough that the difference isn't measurable.