'Willowy' is an aughisky ideal, and since they're not remembering what Dogs were like that well, they're choosing for what they think are attractive.

Tall (long legs, anyway) and good lungs, though are runner's traits, and Dogs are long distance runners, with more endurance that horses.

Iros personally thinks tall and willowy is attractive -- and since Turnspit is tall (for a human), and slim, Iros thinks he's very cute.