I think these guys are so skinny that even small amounts of muscle mass stand out, especially with the skinsuits they're wearing nowadays.One think I've noticed about Primoz is his upper body is much more muscular than other stage racers and most non-sprinters. As a female I must say it is quite attractive. But is it a plus or minus for him as a GC contender?
The weight difference won't be big at all, and arm muscle probably helps some in stuff like sprinting and standing on the pedals.
Pinot probably also has slightly higher upper body mass than a few others.