Why would a trainer skimp on feed? I would only be guessing but have to assume the reason is money, since feed costs are the second biggest expense a trainer has after payroll. That being said, some trainers have very odd ideas about how to manage their horses.
Owners or potential owners should feel comfortable asking their trainer about how their horses are being fed, and definately ask for information about their trainer's overall feed program. I believe that to protect themselves owners should read up on the basic care recommendations for a racehorse in training. http://www.thehorse.com/articles/10331/feeding-racehorses is an excellent article about the feeding of racehorses from a youngster to full training and racing age. We especially like the quote below, because we see far too many racehorses that are underweight and run down. Owners, ask questions - it is your right to know.
"Although a racehorse will perform to his best advantage if he’s not carrying around any excess body weight, there’s a big difference between being fit and being underweight and run down. Far too many racehorses are the latter, unfortunately, and far too many trainers mistake simple ribbiness for good muscle tone! Like any other type of athlete, a racehorse must have stored reserves to run on, or he’ll run out of gas in the stretch. Be sure to provide your horse with enough groceries so that he’s able to maintain some cover over his ribs and spine. If he’s a picky eater, or the nervous type which walks his condition off in his stall, try making his diet more energy-dense by substituting corn or barley or a high fat extruded or pelleted feed for oats or an oat-based sweetfeed. (Don’t neglect regular deworming and dental care, both of which can have a serious impact on a horse’s ability to maintain condition.)"
Another really useful tool we found is a "feed calculator" at http://horse.purinamills.com/products/feedingcalculator/ which includes recommended amounts of "Purina Race Ready", a feed used by many thoroughbred trainers. For a thoroughbred in race training, use "Performance --- very heavy work" for the "lifestyle setting" and 800-900 lbs for the weight of the horse, and the calculator says this horse should have 10-12 lbs of Race Ready feed per day and a similar weight in hay per day.