[beebmam]

The amount of combusted material that one is exposed to makes a big difference in cancer rates (and other disease rates).[1]

I imagine that this is orders of magnitude lower for marijuana smokers than tobacco smokers, especially in this era with most flower being >20% THC by mass.

1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6235683/

[midoridensha]

The reason weed is less cancerous than tobacco is because weed smokers typically smoke far less total volume of plant matter than tobacco smokers. Cigarette smokers light up many times every day. Most weed smokers only use it recreationally on an occasional basis, because it's not as addictive.