The initial interaction of the gamma-ray is a pair production interaction which has a relatively small opening angle for the created pair. On the other hand, the first cosmic ray interaction is some exotic nuclear interaction where many different particles can result, each splintering off in different directions, some at large angles. Further interactions tend to accentuate this. The plots at the learning center are actually hit positions of photons from simulated air showers. If many more events were plotted, and some smoothing were done, the shapes you describe would start to be apparent.
Some observatories use mirrors originally intended for other work - such as solar power studies.
For this type of work, and air Cerenkov work, it is the size of the mirrored surface, not the quality of the mirror which is important. Using the Whipple Observatory example, it would be very expensive to manufacture a smooth mirror with a 10 meter diameter. The shape of the individual facets is not really important. Even gamma-ray showers look circular at the center of the pool. The light intensity in the pool does have an interesting radial dependence.