The speaker begins already stuffed, yet walks out into a land that is just as full, with plenty all around. Instead of stopping, he keeps consuming. The strange, almost comic image of eating "a tired horse" pushes appetite past the point of reason. A horse is a working animal, something we associate with effort and movement, and here it becomes just another meal.
The last lines turn the joke on the speaker. The horse can't "keep pace / with my fat," so the speaker's body has outrun even the animal that might carry him. There's a flicker of self-mockery and maybe some discomfort, as if he sees himself clearly and doesn't quite like it.
The title, "Consumer report," suggests a cool, deadpan review of what a consuming person does. The mood is darkly funny but uneasy, a short, blunt look at excess in a world already overflowing.