That is a sign that he has a plenty of wind.

You take any ordinary horse, and you can't hear him draw a breath; his lungs are frail and he daren't inflate 'em.

But my horse fills his up and blows 'em out again vigorously, so people can hear for themselves how he enjoys the fresh air.

Now, I'll let you into a secret, only mind you don't go to whispering it about: When you want to buy a horse, go and stand off a quarter of a mile and see if you can hear him kinder sighing.

If you can, why go for that horse; he's worth his weight in gold.