These reviews which comment on how wide the Multipla is have puzzled me too, but the following MPVs are all wider, by up to 10cm (according to What Car):
Ford S-Max, VW Sharan, SEAT Alhambra, Peugeot 807, Ford Galaxy, Hyundai Trajet, Renault Espace, Citroen C8, and Citroen Grand C4 Picasso.
The following are also wider:
Audi Q7; BMW 7 & X5; Chrysler Grand Voyager; Citroen C5; Honda Legend & CR-V; Hyundai Santa Fe; Jaguar XJ; Jeep Grand Cherokee; Land Rover Freelander, Discovery & Range Rover; Lexus LS & RX; Mercedes S-Class, CL, R-Class, M-Class & Viano; Nissan Qashqai, Murano & Pathfinder; Peugeot 407 Coupe; Porsche Cayenne; Toyota Land Cruiser; VW Phaeton, Caravelle & VW Touareg; Volvo S80 & XC90. (Not to mention Bentley, Ferrari, Lamborghini, commercial vehicles, etc.)
Certain parts of London would be pretty empty without this lot, so don't worry about the Multipla's width.
I assume that the car 'feels' wider because it's much shorter, and because the driver sits closer to the door.