what defines the number of filemap bits?

So, I've just brought online a new Squid machine.
(RedHat 6.2 with ReiserFS)

With a 24Gb RAID disk (Compaq 3200) and allocating 10 of that to cache
space.
Should I be concerned that I've only filled 137M of that cache and already
used 52% of my filemap bits?

Is this dynamically sized as needed , or is there a parameter somewhere that
I need to tweak?

Thanks,
PhiL P

From Cachemgr.cgi:

Store Directory Statistics:
Store Entries : 17342
Maximum Swap Size : 10485760 KB
Current Store Swap Size: 137048 KB
Current Capacity : 1% used, 99% free

Store Directory #0: /home/squid/cache
First level subdirectories: 128
Second level subdirectories: 256
Maximum Size: 10485760 KB
Current Size: 137048 KB
Percent Used: 1.31%
Filemap bits in use: 17141 of 32768 (52%)
Filesystem Space in use: 211344/24626124 KB (1%)
Filesystem Inodes in use: 0/-1 (0%)
Flags:

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Quokka Sports, Inc.                      Phil Pierotti
Ground Floor,                            Senior Network Engineer 
525 Brannan St,                          phil.pierotti@quokka.com
San Francisco CA 94107                   Ph: +1 415 977 2390
                                         Fx: +1 415 908 1841
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http://www.quokka.com/                         / )o  
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