Actually, a buggy should be stronger. The arms are shorter on the buggy, but not any less thick, so if the buggy and truck are going the same speed and clip a pipe, less leverage is applied to the arm, because they are shorter.
However, the larger diameter, and wider truck tires often absorb or ever ride over the pipes, whereas the buggy will get jammed: breaking the arms or spindles etc.
Then again, a buggy weighs less: smaller tires, shorter chassis, shorter arms, smaller body. Buggys are faster than trucks for the most part, again making them break easier.
That is why most people think the xxx4 breaks all the time. It isn't because its weak, but because generally your going faster through the turns, on the straight, and over jumps (or trying to anyway)