We have a Civi user connected to Mailchimp who has quite a few lists - from our testing (Mailchimp v 1.6, Civi v 4.4.6) the extension appears only to access the 25 most recent lists in Mailchimp. How can this limitation be increased?
The first thing I'd do is upgrade the extension from 1.6 to 1.8 to see if this issue has already been addressed. If it still remains, then I would create an issue at https://github.com/veda-consulting/uk.co.vedaconsulting.mailchimp/issues
Any use of the api without specifying the options.limit return only 25 results. So my guess is the MailChimp extension doesn't specify this option when retrieving the list of groups.
But as mentionned by Joe Murray, it's seems to be a bug in the extension that need to be fixed and an issue has been logged - https://github.com/veda-consulting/uk.co.vedaconsulting.mailchimp/issues/155
So there not much we can do here.
Follow up: This limitation has been removed and should be available in the next extension upgrade. See: https://github.com/veda-consulting/uk.co.vedaconsulting.mailchimp/pull/158/files