My dog is regarded as medium sized by the breed standard so that is the food I buy.  The food stops at 25kg.  My dog is not overweight as her waist is really defined, the ribs feel like pencils in a pencil case and the backbone can be felt but not seen.  She weighs 27kg so do you think I should buy food for a large dog which starts at 25kg in the same brand?  When I read the food amounts to be given they are the same amount as she gets now.

Medium.

I think you'll find all this size-related food means the size of the kibble rather than any difference in the actual food make-up.  Other than puppy vs adult food of course.   The food amounts on the bag are always just a guide.   If I gave the amount recommend for my biggest hound, he'd burst.   And I'd suggest this size related food is also just a guide.    if your breed is a medium breed according to the Breed Standard, and if you buy a size-related food, go with that recommended for medium dogs.    Especially if she manages to eat that size food okay.

You should be able to feel the ribs not see them, so if this is the case with your dog, she's fine.