The size of the IIS Object Cache is not a common problem, but it is worth monitoring because the cache is not efficient if it is too small.
Compare Internet Information Services Global: Cache Used to Internet Information Services Global: Cache Size over time. These are instantaneous values, not averages, so it is important to use a large data sample. If the Internet Information Services Global: Cache Used to Internet Information Services Global: Cache Size ratio is more than .9, and you have sufficient available memory (which you can determine by using Memory: Available Bytes), you might consider increasing the maximum size of the cache to 5MB. Alternatively, if the ratio of Internet Information Services Global: Cache Used to Internet Information Services Global: Cache Size remains less than .2 over time, consider reducing the cache size.
To change the size of the cache, use the MemoryCacheSize value entry in the Registry. It is located in:
HKEY_LOCAL_MACHINE\System
\CurrentControlSet
\Services
\Inetinfo
\Parameters.
After changing the value, restart the service, log the IIS Object Cache counters again, and watch for changes in performance.