I would honestly expect that with modern spam filters the random rate fluctuations of spam getting through the filters would have a bigger impact on the spam you see in your spam folder than the actual number of devices sending it.

Also, if your theory were correct that those make up a sigificant percentage of spam sending devices I would expect home office trends during the pandemic (which often use devices sending all their outgoing traffic over VPNs with relatively strict firewalls) and especially complete closures at the start of the pandemic to have a big impact on spam volume which has never been brought up anywhere to my knowledge.