How is it possible to receive traffic 10 times more from a list that has ten times less members than another?
Many list sizes are inflated. For example, if a site claims 100k list you can mail to, I would bet that 90% of that list is inactive and will never even look at the email.
I can speak for myself that my sites are constantly scrubbed, so you ONLY send to a list that has a RECENT history of reading and clicking on mails. Anyone who is not reading we stop sending them mails, so
as an advertiser you get the most value. Some other lists clean up their member sizes as well so you aren't sending to "dead" prospects, but there are quite a few that will claim the higher numbers just to look good and as a result you
will receive a very low CTR because those people just aren't reading the email.