The blog post is a little over a month old. The installer is 60MB now.

Arc is a daemon that runs on your system, like DropBox. Browsers can use arc.js and instruct Arc to run native code in lightweight Linux virtual machines.

Arc is a stronger approach than maintaining browser plugins. Arc apps, like web apps, are up-to-date every time they're opened. They also have full access to their Linux virtual machine and can use Linux programs as-is. Browser plugins can't provide that.