Zane gave the appearance of being small, but put him next to Bannout, Platz, Dickerson, Mentzer (his contemporaries), and he had more width than them all, invariably he was also taller.
Whilst he appears to be a swimmer, go stand next to him at his prime, and you will notice the thickness in the pecs, and find your own legs to be dwarfed.
All the pics of Arnold & Zane in street clothes, Zane isnt exactly looking much smaller than Arnold.