This is indeed a problem, but can easily avoided without needing to run unnecessarily long trains.
Most importantly, the station needs to announce the size of the train before it arrives, so people can arrange themselves accordingly. Like you said, with a 6-car train, the cab car is 150 metres from the end of the platform, so it could take up to 2 minutes to walk to the nearest door if someone's waiting at the wrong end of the platform. Which is longer than the trains typically stop. Here in the Netherlands, the length of the next train is displayed on the screens, as you can see below. Dutch platforms are also 12 cars long like ours, yet it's no problem that this particular train is only 4 cars long. As in Ontario, you can see the stopping markers along the platform, so you just need to make sure you are between the <4> signs in either direction (you can see the blue <4> sign in the background).