The reason I asked you to check if you had a circular Anti-Pad was if you enabled the Padstack option "Mech pins use antipads as Route Keepout", it would use the Anti-Pad feature defined in the padstack as a Route Keepout to void the plane.
If you don't have Route Keepout defined and have not enabled the Anti-Pad as a Route Keepout in the padstack then the shape void will be based on the drill size using the Hole to Shape spacing check which I believe defaults to 8mils.
PS. As I am typing this, I just saw your response that you found the Sq Route Keepout in the symbol, great! - still thought it would be good to close the loop with this follow-up post anyway.
Hope this helps,