There's a consensus made on the global state of the Tor network made between several well-known parties: just as a regular Tor client, Lighnion downloads the consensus document, parses it, validates its signatures and perform a path selection algorithm on it. It's several megabytes to downloaded by the Javascript client (itself being few kilobytes) and is currently a limitation of the protocol.

If the "walking onions" proposal gets maintstream (i.e. private retrieval of a consensus subset), we can hope for improvements (see https://gitweb.torproject.org/torspec.git/tree/proposals/300...).