These days, most hay is bailed in large bales - usually round, but sometimes square. Sometimes you'll see horses in a field gathered around one.
Most horse owners, however, still use the small rectangular bales. Why?
It's simple. The large bales require agricultural machinery to move. That bale in the middle of a field was probably delivered there. If you have a small to medium sized herd, it's still more effective to use the small bales, which can be lifted and moved by one (fit) person and fit in a wheelbarrow. Most horse owners can't afford to have a forklift around just to move hay.
Small bales also break up easier and if one of them's rotten, you lose a lot less hay.
Image source David Shankbone via Wikimedia Commons.