I tried this as I too have a large size in a production box but it seemed to make no difference I noted the below reference points info seems contradictory ??
In a dev server I tried the delete (zmcontrol stop, deleted the contents zmcontrol start) and so far no ill effects but this was a very limited test with only 2 mailbox imap interaction and only about 80mb in files roughly 50 / 30 split ....
My reference points
https://bugzilla.zimbra.com/show_bug.cgi?id=74478#c1
See the last comment (N0 6) not sure if that helps
And The 8.8.6 Admin Guide:
https://zimbra.github.io/adminguide/latest/index.html
zimbraImapInactiveSessionCacheMaxDiskSize. Specifies the maximum disk size of inactive IMAP cache in Bytes before eviction. By default this value is 10GB. This is a rough limit, because due to internals of Ehcache actual size on disk will often exceed this limit by a modest margin.
zimbraImapInactiveSessionEhcacheSize. Specifies the maximum heap size of the inactive session cache in Bytes before eviction. By default this value is 1 megabyte. This is a rough limit, because due to internals of Ehcache actual size in memory will often exceed this limit by a modest margin.
zimbraImapActiveSessionEhcacheMaxDiskSize. Specifies the maximum amount of disk space the imap active session cache will consume in Bytes before eviction. By default this value is 100 gigabytes. This is a rough limit, because due to internals of ehcache actual size in memory will often exceed this limit by a modest margin.