I’ve always presumed that the “+” button is the most frequently keyed button when you compare it to the usage frequency of the other none numeric buttons on a calculator – hence it is physically larger to make it more available when fingers are rapidly targeted at the keypad. When you look at and Olivetta Divisumma 18 you must remember that the designer Mario Bellini ‘responds to human requirements in his design’ (Tambini 1999).
REFERENCE: Tambini. M (1999) The Look Of The Century – Revised Millenium Edition. London Dorling Kindersley