Index-cache sizes

[Tom Sommer]

I had a customer with an INBOX cache of 400MB+ (5.5GB maildir) which 
gave me "Error: mmap_anon(474165248) failed: Cannot allocate memory" 
errors, then I deleted the cache files and ran 'doveadm index -u 
xxx at example.com "*"', now the INBOX cache is 7MB

What is up with that? Is the cache now missing something or does it make 
sense to wipe cache files and reindex cache every-so-often?

-- 
Tom