It's pretty much impossible to properly import a big XAR with standard tools right now.
See the extensions that already exist on http://extensions.xwiki.org
Introduce framework to stream wiki data from one representation to another
Introduce output instance component for WikiStream
Introduce input XAR component for WikiStream
XAR export/import takes way too much memory
Impossible to export a large wiki to a XAR
Use WikiStream by default in XAR import action