In general, what is the difference between the "big" core and the "small" core? Only the performance difference?

@zecheng the difference is in chip area (number of transistors) and power. The mobile chip here needs to be compact to fit into a phone and the power consumption needs to be low for batteries to last longer. Performance is a rather vague concept as it depends on the workload/benchmark.

Is there a practical limit to how many cores can be on a processor? Why is it that Apple chose the 2+4 design for their A13?