Toronto's timidity at the high end, and Chicago's boldness, is more interesting than the stats. It's also the key factor in this over the short- to mid-term.
Now, Chicago is one of the great skyscraper cities of history. Maybe it's THE great one given New York's fussy setbacks and its own inimitable procession of stark '70s giants.
But Toronto raised the CN Tower (I know, not a skyscraper... but you look at Toronto and
there it is, so...) when it was a small metro of maybe 2.5 million (between
Pittsburgh and Boston, it looks like), as well as the world's tallest outside New York and Chicago. So there was an appetite there, maybe even one larger than it has now despite its growth finally rising to meet its ambitions.
At the top end, it becomes about will. Both cities could go into the stratosphere on the right combination of ego, design and localized demand. But Chicago looks more willing to floor it. Bigger shoulders, maybe. Or balls.