Hello,
SOLR 4 content folder is not filled with what you consider to be "content", that is full text contents of documents being index. The content folder holds compressed JSON files containing index data for all documents / folders. This includes metadata and path information, not just full text data. So even when you disable full text indexing will SOLR create those files but they should be very, very small and only contain metadata, ACLs and such.
As far as I can see, there is no way to disable this behaviour. It is intended as a cache mechanism to avoid SOLR having to re-fetch all the data for a node whenever it has to re-index a document without a change to the document itself (or with only a limited change), i.e. when a path changes due to some folder in the hierarchy being renamed or an ACL is updated.
Regards
Axel