G'day Paul,
bailey is on the money with his figures, one thing you and the young bloke will notice when doing your equations is that no matter what the circumference /diameter of each turn the horse on the outer loses exactly the same amount of ground per turn ie; Menangle approx 2.1 metres per turn/Parkes 2.1 metres per turn per gig width.
The bigger the track the less ground lost per mile per gig width due to the less turns negotiated.
cheers,
Dan