Just for giggles I went and measured my four horses. They are all thoroughbred and 16 hands ish.
Mare in mediums now is 14" in front and 13.75" behind. She is built thick but with wee little feet and light in bone. Mediums look to fit her well.
Next one is 14" both front and behind. She’s lighter than mare one but still pretty solid. Has better feet and bone.
Next one is 14.5" in front and behind. Lean, narrow gelding but pretty solid feet and decent bone.
And the big girl–15.25" in front and behind. Very solid horse with big platter feet.
@alfonsina I just can’t imagine your horse, which is taller than anything I have, measures so small around the coronary.