Two factors come into play. Scattering of light in the atmosphere and glow/glare from the optics capturing the image. Some earlier atmosphere models such as Mental Ray’s physical sky did a big mistake of embedding the glare right into the sky model itself, not just atmopsheric scattering. But that does not seem to be the case here. Just turning of bloom in post processing settings displays the actual sharp sun disc. You will also be extremely surprised how tiny the sun disc really is. Generally, people tend to underestimate how small the sun disc without the glow actually is. That may in fact tie to @The_Distiller 's request above, where he may have gotten giant sun disc mainly due to a glow.
Some examples:
Default new sky with default amount of bloom (no PP volume present in scene to override it):
Convolution bloom for more close to reality result:
