It does look like that, but it may simply have been drawn with those proportions for illustrative purposes. On the other hand, perhaps the diagram has correct

It does look like that, but it may simply have been drawn with those proportions for illustrative purposes. On the other hand, perhaps the diagram has correct proportions but they're misleading: the combustion area might be roughly spherical rather than cylindrical, and we're just seeing the cross section that shows its maximum area. edit: actually, this is covered in their FAQ: > What is the compression ratio? > For the X Mini 70 cc spark-ignited prototype, the compression ratio is 9:1. The diesel prototypes have operated at between 14:1 and 18:1, and should be able to go higher, to 25:1 or more.