Quite honestly I think you've just hit on a particular combination of disc size, brightness and render resolution that produces a square for the intended circle, given the low brightness settings (which affect glow that will usually help smooth out the edges). Although other lights do have the same disc size, it's the combination of that and brightness which currently controls glow strength which is ultimately resulting in this issue.
Imagine for example a small sun that turned out to a 4 pixel disc in the sky at a given render resolution - it would necessarily have to be square; that would be the most correct shape. Circles in computer displays are always an approximation, sometimes a very bad one. This is usually dealt with most effecitvely by antialising. Here the problem may be simply due to the extreme brightness of light sources, which currently isn't handled as well as it should be. So I'm not sure this is actually "correct", it may be a bug, but simply adjusting render resolution does fix/change it, so it would seem to be pretty particular settings that cause this and it's probably some kind of rounding or accuracy issue if anything.
Anyway I'd suggest slightly changing the disc size to fix it for now. You can expect improved antialiasing options implemented in the future to likely address this sort of thing.
- Oshyan