Addressing the second question first (because it's easier): The pipe is probably manufactured by filament winding, and the fittings are probably manufactured by hand lay-up. The different manufacturing processes yield materials of different strengths. To maintain consistent factors of safety, the allowable stresses must be different.

The explanation concerning size is more complicated, but here goes: The manufacturer probably has a minimum thickness for filament winding, regardless of the pressure rating of the pipe. Even though the structure of a 2" diam. pipe could be 0.04" thick, the manufacturer might not use anything less than 0.08". Now, filament wound pipe is best suited to resisting axial stress when that axial stress is as a result of pressure. In fact, the typical 55° fiber winding angle is used because it is the most efficient to resist pressure. This winding angle will result in the fibers being stressed in pure tension (theoretically at least), when the pipe is exposed to hoop stress that is twice that of the axial stress, i.e. pressure. The axial strength of that same pipe is lower when exposed to purely axial stress, e.g. bending due to pipe weight. To yield consistent factors of safety, the allowable stress must be a function of the two axial strengths. In the smaller pipes, which are thicker than necessary for pressure containment, the bending strength plays a more dominant role. Hence, the allowable stress will be lower in the smaller pipes.