A good portion of the size difference (though certainly not all) is that ntp.org's ntpd has a lot of drivers to interface with physical devices [1], while OpenNTPD doesn't. If you don't own the physical devices, the LoC in the drivers are kind of a wash: they're not useful to you, but you also won't be running them. A stripped-down NTP-client-only build option would be nicer, though. PHK's suite of ntpd replacement tools seems to be going in that direction, offering a client-only tool for edge servers that just need to sync time over the network.