Really? My experience has been the complete opposite.
Almost every time I've even been involved with or ran any medium/large mail infrastructures (200K active mailboxes) I've always found Dovecot to be a better choice.
Good to hear that Dovecot scales. I personally went with Cyrus because at the time I was picking an IMAP server, Cyrus was already a well tested solution known to scale. As I recall, Dovecot back then was still young and developed by one person. I've heard a lot of good things about it since though. I'm sticking with Cyrus since it works fine for me, but it's good to have options.
Almost every time I've even been involved with or ran any medium/large mail infrastructures (200K active mailboxes) I've always found Dovecot to be a better choice.