The smart has a two stage braking system. The first is ordinary braking. The second is for panic stopping. Here the car senses that you need to stop fast and increases the braking force significantly. As to your sense of the brake being very hard, it seems to me that is a subjective observation which I really can't comment. I will say that from my experience, the brakes in 'standard' mode seem to be just fine. In 'panic' mode, they are incredible and saved a couple of bikers from becoming hood ornaments for my smart.
Some users have noted the brake peddle feels strange, and unlike other cars they are used to. On first drive I found Smart's brakes to be touchy, but got used to it after a few applications.
The difference in feel is due to the Smart's brake peddle coming up through the floor and not suspended from the dash as in most cars. After you drive awhile you will find the the brakes to be very good. A2Jack