Does anyone know the math of stacking growth effects?

So I’m one of THOSE RPers and I like to make my character as big as I can. I’ve found that you can stack Winterfall Firewater and Darkmoon Firewater for a greater size increase, and I’m wondering if anyone knows how the math of those works.

Winterfall Firewater increases your size by 25% and Darkmoon increases it by 15%, according to the spell effect list on wowhead. But what I’m uncertain of is whether that’s a directly stacking buff, so overall it makes you 40% bigger, or if the second one does a percentage of what size you are when the other one is applied. Is there a way to check this that anyone knows of?

… It’s all natural!