The width of the cars would be related to the compact inner city "fit".
The ALRT was designed as an elevated
inner city rapid transit system (i.e. Detroit Peoplemover)
A key feature is the ability to make sharp turns
around buildings within a city grid system.
The features that allow that are the short cars and the steerable axles.
The turning radius was a huge feature - exemplified in Vancouver with the sharp turn into Main St. station and in Toronto with the (now decommissioned?) turning loop at Kennedy.
Remember, that was in the days before fully articulated trains.
That's also the same reason why LRTs often have many articulations - to allow cornering.
The Bombardier ALRT seems to have moved away from the original concept with the longer MKII cars (i.e. based on actual implementation)
For rapid transit cars dimensions, see here:
Elevated lines in NYC and Chicago also have short cars.
http://greg-vassilakos.com/traindwg/traindwg.htm
Chicago El - Big City - short cars!:
http://en.wikipedia.org/wiki/Chicago_'L'
The other factor would be whether oncoming cars "clip" each other on tight turns (on sections of dual guideway) due to the tight radius. I'm not sure if there are any, though, as I think the guideway splits for Main St. station at that turn.