You can stretch and pad the 225 as much as you like but the girth will remain the same.
The 225 shares the same basic fuselage as its predecessor, AS330 Puma, which was built to a French Army requirement. Two of those requirements were:
To fit into the back of a C160 Transall.
To fit on a flatbed SNCF truck.
For the Transall there was a portable crane that attached to the side of the aircraft that enabled a team to remove the engines and gearbox in 30 minutes. A trolley arrangement that the aircraft was pushed over and then the mainwheels retracted so that the pylon canted down as it went in the back. A feature similar to the present 'kneel' facility and used for the same purpose.
For the SNCF it had to be narrow. Fans of armoured vehicles will know that French troop carriers of the period were famously high and narrow for the same reason.
I would support the reasoning that there was no point in designing a larger fuselage. It hits the military market, the S92 does not have one, and you are only adding space that is not used. I have spent a lot of time on operations where I am in direct social contact with my passengers and I have never received any complaints about lack of space, trust in the aircraft or anything else.