"Processed meat was classified as carcinogenic to humans (Group 1), based on sufficient evidence in
humans that the consumption of processed meat causes colorectal cancer."
The groups have nothing to do with "ranking", they're based on the amount of evidence that we have for the link. From the IARC's FAQ:
> The classification indicates the weight of the evidence as to whether an agent is capable of causing cancer (technically called “hazard”), but it does not measure the likelihood that cancer will occur (technically called “risk”) as a result of exposure to the agent.
So this tells us how certain we are that they cause cancer. And we are certain that red meat causes cancer, and we are certain that smoking causes cancer.
This ranking does not tell us how much cancer is caused. We know that there is a low risk of colon cancer, and that eating red meat will increase that risk by about 15 to 20%. This ends up at a very low risk.
Cigarettes massively increase the risk of lung cancer. (And other cancers).
> The classification indicates the weight of the evidence as to whether an agent is capable of causing cancer (technically called “hazard”), but it does not measure the likelihood that cancer will occur (technically called “risk”) as a result of exposure to the agent.
(Emphasis theirs)
http://monographs.iarc.fr/ENG/News/Q&A_ENG.pdf