my cache is small!

We've been running our 1.0.18 cache for about a week and it swelled to
about 90Mb (on 1.4Gb cache) and now it's shrunk down to a piddly 70Mb.
I'm running with vanilla config files but even that shouldn't mean I
should suffer from such a small cache should it?

I checked the FAQ/Rel notes etc and wasn't able to glean to much from it
about how to increase the cache or how to decipher store.log if that is
what I should be looking at.
I've also uncommented the TTL lines but to no avail.

Anyone able to help?

Regards,
   Graeme

--
Graeme Merrall
System Administrator/Web Spinner
Manawatu Internet Services Ltd, Palm Nth, New Zealand