initialSize of queryResultCache and documentCache

[Phillip Farber]

I'm trying to understand the purpose of the initialSize parameter for 
the queryResultCache and documentCache.  Is it correct that it controls 
how much heap is allocated to each cache at startup?

I can see how it makes sense for queryResultCache since it is documented 
as an "ordered lists of document ids (DocList) based on a query, a sort, 
and the range of documents requested".  Therefore the initial size can 
be computed.  But the documentCache is a list of stored fields and 
fields vary in size so how is the initialSize determined?

Cheers,

Phil Farber