queryResultCache

[s d]

What is the best approach to tune queryResultCache ?For example  the default
size is: size="512" but since a document id is just an int (it is an int,
right?) ,i.e 4 bytes why not set size to 10,000,000 for example (it's only
~38Mb).
I sense there is something that I'm missing here :). any help would be
appreciated.
Thanks,