Can you combine a Polymorph spell with size increase effect?

[Goznaz]

I’m well aware you can’t combine a Polymorph spell with a size increasing spell, but i’m curious about supernatural abilities. Specifically I’m wondering on the combining of Form of the Dragon 1 with the Orc Sorcerer Bloodline (SU) ability power of giants.

“At 15th level, you may grow to Large size as a standard action. At this size you gain a +6 size bonus to Strength, a -2 penalty to Dexterity, a +4 size bonus to Constitution, and a +4 natural armor bonus. You may return to your normal size as a standard action. You may remain in this size for up to 1 minute per character level per day; this duration does not need to be consecutive, but it must be used in 1 minute increments.”

Thanks in advance for any and all help with this and apologies in advance if my Google fu has failed me.

[deuxhero]

Polymorph Subschool Description wrote:

You can only be affected by one polymorph spell at a time. If a new polymorph spell is cast on you (or you activate a polymorph effect, such as wild shape), you can decide whether or not to allow it to affect you, taking the place of the old spell. In addition, other spells that change your size have no effect on you while you are under the effects of a polymorph spell.

Note that Long Arm is not a polymorph or size change effect, so it does stack.