My /var/spool/mail has grown drastically. Is there any possibility of using two directories?

You can use an expansion string to split mailboxes between two directories. For example,

file = /var/spool/mail${nhash_2:$local_part}/$local_part

which does a hash on the local part, producing either 0 or 1, thereby using (mail0) or mail1. But remember, the MUAs that read these mailboxes also have to know where they are.