Weapon Reach & Creature Size?

[Wolffenjugend]

I've got a large creature mounted on a gargantuan creature. The large creature has a (natural) reach of 10 feet and is using a Spear. The gargantuan creature has a reach of 15 feet.

According to the PH, a Spear has a reach of 10 feet. I've adjusted the Spear's damage due to the Large size of the creature, but does the reach of the spear increase (i.e. to 15 feet) as well? I can't find anything in the PH or MM.

The specific reason I'm asking is b/c the mounted creature will be charging and I need to know how far away the gargantuan creature needs to move/stop.

 

[Staffan]

IIRC, reach weapons generally double the wielder's reach. So, if the guy with the spear normally has a reach of 0-10 feet, he now has a reach of 11-20 feet.
The exception is for Tiny and smaller creatures that normally have a reach of 0, who get a reach of 5 feet with a reach weapon.

Also, it's possible that the rider "absorbs" the entire space of the mount. I am pretty sure a human on a horse uses the horse's entire 10x10 space (and can attack any adjacent square), so it might work that way for larger riders too.