Once tamed, they may be steered, but their natural tendencies still have to be have to be reined in. People do that by saddling and bridling a horse. The reins are connected to the bridle, and specifically a bit is placed in the mouth of the horse, over his tongue. If the rider pulls up on the bit, the head of the horse raises, and he slows down. The tugging of the small bit, one-thousandth of the weight of the horse, can bring a large stallion to a halt.
Small objects can have great impact.
The second illustration in v. 4 is of a ship. Think of the kind of boat that the apostle Paul sailed on, carrying hundreds of people. This large ship, set afloat on a sea, would be quite difficult to turn except for its rudder. With roughly the same proportion as a bit to the total size of a horse, a small rudder can be controlled by a pilot and the ship is steered by that mechanism. A large boat could not possibly be controlled by even a large number of oars.