A little over 50% of a roof tile is covered by other tiles to provide the weatherproofing. The square solar part covers nearly all the exposed area.

From when I was reading more about solar cells I recall there being something about the distance from the conductor (the two stripes visible equidistant from the midline of the cell) being a problem especially with monocrystalline cells, which these appear to be. I think this was more a problem for cells that had the conductor fixed to the back of the cell.

Google image search for monocrystalline solar cell and you'll see they are all square. I had a brief go at searching for anything to back up my claim but lost interest.

I might simply be that the silicon ingot they are cut from is cast round, see this[1] picture and it's less wasteful to cut a square with chamfered corners than to cut a rectangle, which would necessitate an ingot with an elliptical cross section, which would probably be harder to cast due to uneven cooling, maybe.

1. http://www.tindosolar.com.au/learn-more/poly-vs-mono-crystal...