@Aibohphobia will be able to answer that, but you pose an interesting question.
I was also looking at that board today. Just out of curiosity I looked up some examples:
The "Asus X99 WS" is a "
CEB" form factor, according to Asus it's 12 in x 10.5 in (305 mm x 267 mm).
Boards like the "Asus Sabretooth X99" are
ATX form factor, 12 in x 9.6 in (305 mm x 244 mm), so the CEB board is "only" 23mm wider.
Dual socket boards like the "Asus Z10PE-D16 WS" are "
EEB" form factor on the other hand are 12 in x 13 in (305 mm x 330 mm)
It gets all fuzzy with E-ATX apparently:
Wikipedia said:
Although true E-ATX is 12 × 13 in (305 × 330 mm) most motherboard manufacturers also refer to motherboards with measurements 12 × 10.1 in (305 × 257 mm), 12 × 10.4 in (305 × 264 mm), 12 × 10.5 in (305 × 267 mm) and 12 × 10.7 in (305 × 272 mm) as E-ATX. While E-ATX and SSI EEB share the same dimensions, the screw holes of the two standards do not all align; rendering them incompatible.
When I look at a comparison website and filter for motherboards with Socket 1151, 2011-3 and AM3+, I see the following distribution:
ATX 249 (137)
µATX 189 (17)
EATX 33 (24)
mITX 32 (8)
EEB 20 (0)
thin mITX 8 (0)
CEB 4 (2)
Between brackets are when I filter for X99, Z170, 990X or 990FX chipsets. So there are almost 10 times as many ATX and EATX boards combined as µATX boards for high-end configurations.
Just something I was looking into.