Kidd and especially law have 3B without any territories and meh crew though.
Which suggests that, as expected, Yonko bounties are largely due to strength and a smaller but significant portion is due to other factors
Plus, it seems Yonko bounties get higher just with time. Buggy has all the reputation in the world to be a 4B+ Yonko yet they started him at 3B. Teach similarly started at 2B and shot up by seemingly doing not that much