Clovertrail is not 5.6mm2. Hans just cropped the entire square section of the chip while bus unit is smaller than what he suggests. Your second picture on the white highlighted section points out the core+bus which turns out to be 5.0mm2, based on the information that CPU+L2 on PCWatch's pic for Silverthorne is at 14mm2. That's not relevant to figuring out Airmont core size though.
Let's see how you calculate the core size
1. According to PCWatch picture, the Bonnel core + L2 = 14mm2
2. Because the image is according to scale, you find out that Silvermont + L2 = 8.5mm2
3. Take this picture:
http://www.expreview.com/39814.html
4. Now pixel-count based on that Silvermont core there is 8.5mm2. You end up with...
2.1mm with BIU for 2 cores
1.67mm w/o BIU for 2 cores
That's 0.835mm2 for one core. They probably have more room for optimization in Goldmont. With Silvermont we got practically same core size as Bonnell despite having 50% better IPC.
A15 probably has more transistors.
Irrelevant. A15's IPC is approximately on par with Silvermont. Therefore, its right to compare them. Who cares about architecture and ISA idiosyncrasies? Really, only products matter.