It is a little vague but if I am understand this correctly if you can target a mount you can target the rider?
What if the mount is bigger then large but the rider medium or two sizes smaller?
Target a mount or the rider as you desire provided you can see them. There are no problems with targeting the rider unless the GM assigns a cover bonus to the rider (which is possible).
- Gauss
Normally it's assumed you ride a mount 1 size larger than yourself, which is what the mounted rules were written for.
Normally that works fine and is an easy enough simplyfication to make gameplay easier.
But if the mount is now abnormally larger than the rider, like a halfling riding a T-rex (Gargantuan), then as a GM I would most certainly apply some cover bonus to the halfling or actually designate a square where the halfling is to see if you can hit it.
It just doesn't make sense that you can only see a single squre of the T-rex and can hit the halfling, and someone else sees another square at the exact oposite end and can hit the halfling too.