I think optical excellence rather than size was the priority for both Nikon and Canon's new mirrorless lens offerings. They'll probably release smaller (slower) versions later, after the well-heeled early adopters buy first.
I read somewhere recently that the "mine is bigger than yours" mindset is still a significant factor in consumer purchases. Also explains why Canon's white L lenses were desirable, apart from their optical characteristics: they stood out more.
just give me good light and a roll of film