For years, Norman Casse has watched the expansion of his waistline without giving serious thought to going on a diet. "I never had any motivation," he said.

But in the last three weeks, Casse has trimmed 14 pounds with a method that is probably sounder than any of the ballyhood miracle weight-loss systems. He found the proper motivation: money.

Last month, Casse flew from his Ocala, Fla., farm for a day at the races in the company of two other horse breeders, Harry Mangurian Jr. and Bob Gaudio. On Mangurian's six-seat private plane the three men kept bumping into each other whenever they tried to pass through the aisle or the doorway, a consequence of the fact that their combined weight was nearly 750 pounds.

"We all need to go on a diet," Mangurian observed.

Casse scoffed.

"Each of us could put up $5,000 on a bet," Mangurian suggested.

Casse suddenly felt an incentive.

"I'm ready," he said.

On New Year's Day, the three met to establish the rules of the wager and conduct the official weigh-in. They agreed that the man who lost the greatest percentage of his weight during the first three months of 1979 would collect $10,000. Then they stepped on the scales. Gaudio, a former Cleveland Brown who owns a small horse farm, weighed 300. Mangurian, the furniture-store magnate who owns the champion mare Desert Vixen, was 220. Casse tipped the scales at 227.

After the weigh-in, the men attempted to start the year on a virtuous note and played a game of basketball together. "We almost killed ourselves," Casse said.

Some of Casse's acquaintances were a bit surprised that he agreed to the wager because the odds appeared to be against him. It ought to be easier for a 300-pounder to lose a percentage of his weight than for a 227-pounder. And Casse bets only when he has an edge. He has been known to ship horses to such outposts of civilization as the Great Barrington Fair in western Massachusetts so he can maneuver them into spots where he can cash a bet.

(Once he came into the paddock wearing a T-shirt emblazoned NOT TODAY, and his horse was trounced. When he saddled the horse the next time, his T-shirt read TODAY'S THE DAY, and it was.)

Casse knows when he figures to be a winner, and he figured that he had all the edges in the great diet wager. "Mangurian travels a lot, leads a busy social life and has a lot of formal dinners to go to," he said. "Staying on a diet is going to be tought for him.

"As for Gaudio, we were driving together not long ago and passed a Seven Eleven store and he's addicted to them. He stopped and got two Tabs and two polish sausages and said, 'If you eat one, I'll eat one.' Two hours later he stopped in another Seven Eleven and bought six bananas and before we were home he'd eaten five of them. He asked me, 'They're not fattening, are they?'"

But Casse has managed to stick to a Spartan regimen. He walks horses in the morning, then goes home to eat half a sandwich for lunch. At night he might have a dinner consisting of filet of sole, salad and water. Then he walks two miles from his home to the local dog track, bets a couple of races and walks back. Before he goes to bed he does a series of sit-ups.

This is not easy for a man who used to eat three or four candy bars a day, but Casse says his resolve is still strong.

"I won't weaken," he said. "I just never had any incentive before. But I calculate that I'm earning $111.11 a day with this diet."