If she's not due to foal, then she's definitely suffering from an expanding waistline. Now, we all like to see our horses in good flesh, but it's possible to provide too much of a good thing. For some equines, it seems extraordinarily easy to become overweight. Ponies, in particular, have a problem because they usually evolved in very harsh conditions and thus were designed to get the maximum nutritive value out of very sparse, coarse forage. As a result, their metabolisms are unusually efficient.
Several factors can combine to create an overweight horse.
Sometimes, chubbiness is the result of misplaced owner affection. Many of us get considerable satisfaction from making our horses happy, and one of the things that makes them happiest is food. Finally, in a group feeding situation, there always will be horses which are dominant, and those which are submissive. Dominant horses get to eat first, and often will bully more subservient pasturemates into giving up their portions. So, you might be able to identify the boss mare in your field simply by her proportions.