Horses with Cushing's don't usually show all the symptoms that can be attributed to Cushings--they generally show a few of them. So even if you don't think she fits the description you could still be dealing with Cushing's or Insulin Resistance.
Loss of muscle mass in horses with IR or Cushing's tends to be along the topline--they often have plenty of muscle elsewhere--and that loss of topline leads to a pot-bellied appearance.
Anyway, it is something for you to consider and keep in mind to bring up with your vet if you see problems. www.thehorse.com/pdf/aaep/obese.pdf
Is a good article for you to read