[Dovecot] Limiting the size of index files

Is there a way to limit the amount of disc space dovecot will use for its index cache? Other than putting that on a separate partition, I mean.

We host our mail data on the file server, however, the index files are located on local discs on each of the mail servers and seem to grow without bounds. We're above 250M now and soon /var will run out of space.

Thanks for any help.