Makes no sense. Do you have evidence that Xeon E5-2699 V5 is a 28 core SKU? I bet you don't, means your evidence is meaningless. It is a native 32 core Die according to Computerbase, based on informations they received from events like IDF or Hotchips, they are saying that a bigger SKU than 28C was no surprise behind closed doors. Now think twice.
Nope, but 28C is the largest core count on samples shipped earlier. Now, if it is indeed 32C, great. But so far the only evidence i see is hearsay and Chinese folks claiming that.
You don't see "Tick" chips with 6xxmm2 die size. It's always for the "Tock" chips. In that sense, 14nm has quite a bit of room to go considering Broadwell EP is 456mm2. Haswell EP was 660mm2. In contrast, Ivy Bridge was rather large at 541mm2. The growth on 14nm comes with Skylake. 6xxmm2 sizes were traditionally reserved for Itanium and Xeon Phi chips, but they weren't on a bleeding edge process either.
Considering that Skylake XCC has more PCI-E lanes, another interconnect, 2 more memory channels, having 8 more cores on top of that would presumably increase the die size way beyond land of reason. I mean, replacing a DDR3 controller with DDR4 one and adding 3 more cores added a whole 120mm^2 of die size. I would imagine adding 8 more cores would add similar amount of die size. And then we have MOAR uncore to work with.
On the other hand, presumably L3 Cache in Skylake XCC is also much smaller, so maybe that does make it reasonable.