While discussing build options with a fellow party member (an Alchemist), they mentioned a rule granting breath weapons increased area when the user increases size category. After a bit of searching, I was able to find the rule in question:
Breath Weapon: The monster gains a breath weapon that deals 1d6 points of damage + 1d6 per CR. A target can attempt a Reflex saving throw to take half damage. If the breath weapon is a cone, it’s 30 feet long, increasing by 10 feet for each size category above Medium, and decreasing by 5 feet for every size category below Medium. If the breath weapon is a line, its area of effect is twice as long as a cone would be.
However, said rule was located in a section about monster creation/advancement, and now we're debating whether or not it applies to PC's changing size due to spells or buffs. Is there any other RAW indication whether or not gaining or losing a size category would generally affect breath weapon areas in this manner, or would this ultimately be a matter of GM fiat?