Alright, we know that when mounted, you count as the mount's size for purposes of areas you threaten and spacing. Its a simplification.
Now, does this mean similarly that a spellcaster, sitting astride a say Colossal dragon, now occupies colossal space and therefore provokes from potentially dozens of opponents when casting?
It seems pretty damn mean from a RAW perspective, but thats how I keep reading it.
It also has the sadly ironic thing of casting from a dragon's back would also incur concentration penalties for a moving mount, at least how I'm seeing it, correct?
EDIT: FOr some reason this one ended up in Advice. Can someone shoot it over to Rules Questions?