I PM'd Chris about packing assets in Xenonauts for all targets in order to address this problem upstream.
One solution to efficiently update is rsync, which OSX already ships. Only catch is we'd need an rsync server, and rsync is inefficient with lots of clients syncing the same large trees... but then again I can't imagine there's that many Xenonauts Mac beta users. In that case, I'd be happy to volunteer an rsync server: 100 megabits, US, IPv4+IPv6. No 4 GB limit either. PM me if you're interested.