Atlanta and Houston are way smaller than Toronto no matter what the "metro population" count is. Those cities are way less dense and sprawl out forever. I've been to both of them and they are no where close to the size of Toronto in any way.
It's because what counts as the Metro area in a US city is always much larger than what we call the Metro area in Canada.
Greater Houston is an area 26,000km large! Greater Toronto is only 7,000km so to do an accurate comparison for city size, you would have to include Hamilton, Niagara Falls, Burlington, Kitchener etc into the GTA.
If you did that, then Toronto would be much more populated (because it is)
It's a pet peeve of mine when people compare Toronto to much smaller US cities because they don't realize they count an enormous area as the Metro Area.
The best comparison to Toronto in the US for size and population is Chicago, without a doubt. Sadly though, they have a better subway system than us too.