The longer arms twist the axle less per unit of force, but have greater leverage to twist the axle with. It's win-win. This aspect is really no different than tuning a race car's sway bar.
Ok I'll see you, your U-Joint and bushing point of view and raise you this.
Your above statement is almost redundent. What I ment earlier by saying ("but longer arms dont allow for more twist, they twist the axle less which increases movement") is that the long arm changes its relative angle to the ground and axle less while moving. This means that while a short arm moves 10" and 20* a long arm that moves 20" may only move 10*. Which in the end will twist the axle less.
Im not sure I've said that right, but what the heck.