Easy keepers tend to be found most often in breeds originally developed to survive under harsh conditions.
Things like workload, and turnout time, can influence the amount of food a horse needs.
If overfed with a too rich, modern diet, the easy keeper is prone to obesity and other health problems, including laminitis and metabolic disorders.
Easy keepers with a susceptibility to various metabolic problems are also more prone to develop a "cresty" neck, and to not lose weight in that area, even when placed on a diet.
Proper nutrition for an easy keeper involves restricting the diet to mostly a low-NSC hay or very time-limited amounts of grazing on healthy pastures.