To me, those moments of sharing joy through food are way more important than all fries in the world :3
With that said, think ahead and order more than you plan to eat! Outplay her!
I prefer the feedback method: the moment my fries come stick my entire mouth chin and jaw into the bag. I dare anyone to touch my slobber fries after that.
JOEY. DOESN'T. SHARE. FOOD.
My wife explained to me that she gets a small because everything else is enormously sized. Were she to get her own fries, even a small wouldn't get finished. It's nicer for her to share my food because then she doesn't have to worry about wasting what she couldn't finish. Plus ordering a large meal for us to share costs less than an average size and a small. Took years to get over being possessive of my food though.
Give her a break, she earns 25% less than you.
She probably made you pay for her meal anyway.
