ath79 is the successor of ar71xx. It's modernization under the hood, with the main goal to bring the code into a form that is acceptable for Linux upstream, so that all (most) of the whole ar71xx supported devices can be handled by an upstream, unpatched Linux kernel.1)
There might be a slight decrease in kernel image size, but the general bloat of newer versions will likely eat that up completely.
Currently (August 2018) the ath79 target is a source-only target. This means there is no image build by buildbot on trunk/master.