Phantom Steed and casters who gain weight

[Patlin]

No, my sorceror/knight phantom doesn't need to go on a diet, in spite of the title.

I will soon be gaining the polymorph spell, and eventually the stone body spell. Would changing from human form to stone giant form and/or trippling weight by casting stone body discomode the phantom steed in any way?

The spell says:

"It can bear its rider’s weight plus up to 10 pounds per caster level."

So in part what I am asking is whether weight gained through the above spells is still the "rider's weight."

I suppose I'm also asking whether a size large phantom steed can be ridden by a size large giant. Presumably size small characters are unpenalized by the steed being an unusually large mount for them.

 

[Notmousse]

As long as you remain medium size there should be no problems. However, a creature must be willing and able to serve as a mount. Part of being able is being at least one size category larger than the rider, so your ideas of becoming large on a large steed just wont work.

 

[Patlin]

If you attempt to ride a creature that is ill suited as a mount, you take a -5 penalty on your Ride checks.

So you figure a mount of your size is simply disqualified, rather than suffering the penalty for being "ill suited?"

Edit: Nevermind, I found the rule.... Thanks!