It isn't really surprising that the growth numbers are much higher in the suburbs as they have lots of vacant land that can be built upon.  The cities can only grow population-wise by densifying and the growth rates will not look so impressive when you have a huge base already.  Toronto has 2.8 M people in the city proper whereas Milton, ON (mentioned in the article) has 85,000.  So 1% growth in Toronto means that it is adding 28,000 people while 10% growth in Milton means that they are only adding 8,500 people.   So which is growing faster?  Beware of percentages vs absolute numbers.