To be fair to Greater Sudbury, its skewed number is because the sprawl there isn't just a bunch of subdivisions on big lots, it's a bunch of villages all over the valley and Statcan's method of determining the boundaries of urban area include a lot of bush. Thunder Bay is the same, our "urban area" includes a part of the city that is 75% forested and will never
be developed. It fucks up the numbers significantly, as it increases the Urban Area's size by about half.
And as Someone123 said, Thunder Bay's worst sprawl area (where all the houses are on one acre lots) isn't anywhere near our urban area, so that isn't included. But a shit tonne of never-to-become-suburban forests are.
In this map, the red line is city limits, the yellow line is the urban area limits according to StatCan, the pink area within the yellow line is the city's development zone, the pink area south of the river is a bunch of estate lots and subdivisions not included in the urban area, the green areas are major parks and conservation areas (plus a couple wetlands that can't be developed) and the orange is the airport.
Our suburban areas aren't any less dense than, say, Kingston or Winnipeg's. Our urban area as defined by Statcan simply happens to include a shit load of forested areas and rural housing that will never
come part of the city. It would be like including all of Caledon, King, and other suburban towns in Toronto's urban area, and then using that boundary to say "Toronto isn't dense."
Even the most ambitious plans developed in the history of the Lakehead, which included subdivisions and lots on many parts that are within the urban area but will never be developed, did not cover as much land as Thunder Bay's Statcan defined urban area does.