It's hard to judge by looking at them... esp. from the stand. Sometimes only 1/2" or so is all it takes... You just have to wait for a big old boar.... one that makes you say... whoa.... and then keep your fingers crossed.
http://i3.tinypic.com/166ytuc.jpg
The skull on our left measured 18 & 11/16"... The skull on our right measured 19 & 3/16".
Congrats on the kill. He is a beautiful animal.