The Henneke scale is widely used to measure the body condition of horses, with a 1 being "emaciated" and a 9 being "extremely fat". Neither extreme is desirable, and the average pet horse should maintain a 5 with adequate food and exercise.
The score is based on the fleshiness around the neck, withers, shoulders, ribs, loins and tailhead. Tonka would have scored a 7 when he arrived and Raven would have scored a 3 -- as of today I would say that both score a 5, with just a bit of rib visible under their summer coats. For me these changes are most noticeable in their rumps. Tonka's used to be heart-shaped, with hills of fat rising from either side of his spine. Raven's used to be spade-shaped, with valleys between her spine and hips. Now both are perfectly plump and round and shiny -- is it wrong to spend so much time ogling your horses' bums?