Each A13 Lightning core has around 2.5x the transistors of a Skylake core. The Skylake design is also 5 years old and on a node at least a generation out of date. Geekbench is also a short enough test that thermals aren’t a huge concern.
Also, and I’m not sure how big of a deal this is, but Zen 2 is the 2nd gen of a practically new arch. Lighting has a lineage of at least 6 generations.
For A13 with unified L2, I used L2 per core.
Of course, transistor density isn’t consistent across the chip, but this should be good enough for the relative comparison I’ve done.