Your findings are partially right, but you mixed up two completely different setups. maildrop is part of the courier based setups only and not required or related to dovecot setups and the same is with the file ispconfig_mailsize which is for courier only too.
Dovecot does not support traffic accounting yet and the sieve filtering language which is part of dovecot is not able to write files to the filesystem with traffic information as well. So its currently not possible to get the traffic information from dovecot as there is no such log file. As far as I know, the dovecot developers have planned to add a interface for that in future.