Tom Petersen said in a hardware unboxed video that they only reported “active” transistors, such that there are more transistors in the B580 than what they reported. I do not think this is the correct way to report them since one, TSMC counts all transistors when reporting the density of their process and two, Intel is unlikely to reduce the reported transistor count for the B570, which will certainly have fewer active transistors.

That said, the 4070 die is 294mm^2 while the B580 die is 272mm^2.