Sosidge, you are confusing two things - what you see on your car and the Laws of Mechanics and Physics. I hope this helps...
In a 'proper' ARB, the arms don't bend. In a model car ARB, we use piano wire and, as you say, the arm does bend. Ignore this! Now go through DCM's good explanation and you will see the reason for a softer action from a longer lever.
The wishbone moves a fixed distance. If the lever is a long way from the centre section, it will move teh lever a small amount, and thus twist the centre section a little. Move the lever closer to the centre section and for the same movement of the wishbone, you will twist the centre section a lot. More twist tries harder to lift the other wishbone, thus making the front end 'stiffer'
By the same token, make the bar thicker and you will get more lift on the opposite wishbone, again making the suspension 'sitffer' at that end.
Always remember that an ARB is transferring weight across the chassis, and taking grip away from the end the ARB is attached to. Your first port of call should always be the roll centres, in order to change the roll-rate of one end or the other. ARBs come into their own when one can no longer get the desired result from this approach (most cars have limits as to where the suspension pick-up points can be placed) or when the car needs to have suspension capable of absorbing bumps, yet remain stiff in roll.
HTH
