I've had four full sized adults in my 4dr Hardtop several times for short trips around town no problem. Two of us are 6'2" and are over 200 lbs. One of the passengers is generous sized person too. Everyone fit and the car was more than up to the task. In fact, I was impressed that the suspension didn't complain a bit.
Maybe BMW is just 'going retro' on another old British car that they probably own the name to, the Austin A35. When made into a four-door car, it had the most ludicrous rear door openings - you could only get your foot though the door opening by turning it sideways, but then you couldn't get your bum in as well, due to the massive wheel arch intrusion.