This doesn't work with Internet Explorer, so you'll have to use a different browser.
Set the marker anywhere in the city, set the time of day, day (weekday, Saturday, Sunday), if you have a bike or not, and how far you're willing to bike/walk, and it shows you all the places you can get to by PT + walking (or cycling if you set to bike) within that time. Cities with slow transit and/or poor service frequency (total time includes transferring time from one transit route to another) will have that reflected with less total land area covered.
For Vancouver, if you keep the default settings, slide it to 30 minutes, and set the marker to Cambie and 25th Avenue, you get a lit up land area of maybe 55 square miles, which is impressive when you consider it doesn't include any areas that would be reached by commuter rail (you can't get from Cambie and 25th to anywhere commuter rail serves in 30 minutes).
For Toronto if you put it to Yonge and Bloor, 30 minutes, with default settings, you get a land area covered of maybe 50 square miles, only a miniscule fraction of which is by commuter rail.
For NYC, if you set it to Union Square (you can see I'm trying to set the marker away from commuter rail stations for each city to see how much an area you can reach by frequent transit as opposed to commuter rail that might run once or twice an hour) and 30 minutes, you get a covered land area of maybe 42-43 square miles (I'm guestimating based on Manhattan being 23 square miles).
For San Francisco, if I set the marker to the 4th Bart Station from the Bay, at 30 minutes I get maybe 75% of the city, plus parts of Oakland, Berkely, Daly City, and San Bruno.
I set it to Clyborn and Division in Chicago (again, trying to get away from areas that would be reached by commuter rail). At 30 minutes from that location, you can't reach a large area in the city of Chicago, but can reach some good sized sections of relatively distant suburbs, so I assume there are express METRA trains that skip closer-in stops.
For Calgary, from 7th Avenue and 4th Streeet SW, you can reach an impressively large area in 30 minutes when you consider that the only rail there is relatively slow at-grade light rail.
For Seattle, from 4th and Pine you can reach a pretty good-sized area in 30 minutes (though some of that is in Tukwila/Renton, presumably by commuter rail or peak-hours only express buses). To reach almost all of downtown Bellevue from 4th and Pine though, you have to set it to 36 or 37 minutes. Maybe East Link will fix that.