I presume the circle in the icon is not so much meant to represent a Voidsent per se, but rather the portal to the Void the Voidsent is summoned from.

As Iscah said, Job icons are meant to be a simplified stylization representing the lore and weapon of the Job - the class icons are ironically more detailed depiction of the weapons or abilities they use - and as the Voidsent RPR uses is not a pet but an extension of the weapon, the icon makes sense.

Or at least, to me it does.