The first thing to look for on any device you put in the horse's mouth is the contact area. The thinner the bit, the less contact area it has and the greater the pressure on the bars. The effective size of the mouthpiece is the first thing to look at because it will determine how noticeable the pressure you apply will be. The second item to look for is whether the mouthpiece is straight or shaped to relieve pressure on the tongue. If the bit is straight, the horse's tongue absorbs some of the pressure and the horse will feel less pressure on the bars.
Leverage is the third bit aspect to examine.
Most curb bits have a three to one ratio. That means if you put 10 pounds of pull on the reins, the horse will feel 30 pounds of pressure squeezing his mouth. Their pressure is felt as a clamping between the horse's chin and the bars of the mouth. Therefore, it can convey minimal direction to the horse. If you use a chain, the pressure is more noticeable underneath the chin.