If looking for another comparable city I would suggest Houston: roughly 2.2 million city pop / 5.8 million metro pop - almost identical to Toronto.
Breaking down the stats (according to Emporis) Houston has:
2 towers over 300m;
12 over 200m; and
18 over 150m (including 1 u/c);
359 total highrises built, plus 46 planned and 7 u/c.
vs. Toronto with:
1 over 300m (if you include CN);
16 over 200m (9 built, 7 planned or u/c);
31 over 150m (15 built, 16 planned or u/c);
1,823 built, 275 planned, 96 u/c.
But, of course, aside from population there's numerous other political/economic/cultural factors that go into why any city has x-number of buildings of y-meters in height, and most such comparisons I find largely pointless.
Still, whenever I find myself in envy of other cities' current loftiness, I console myself somewhat with TO's historical height prestige. Lets not forget that when First Canadian Place was built (1975) it was the tallest office tower in the world outside of New York and Chicago. And when TD Tower went up in 1967 it was taller than any building in Chicago, period (although still shorter than Prudential Tower in Boston and, if you include spires, Terminal Tower in Cleveland - and neither of those cities are significant players in the realm of "mega-skylines" anymore). Compared to Toronto, most American cities (and world cities for that matter) are relative johnny-come-lately's to the tall tower game.
As far as the future goes, assuming Downtown eventually runs out of space, I'd love to see some sub-super (200-250m), if not super-talls start going up around the suburban nodes like North York Centre, Scarborough Centre, Etobicoke Centre, and along the DVP Corridor. Won't hold my breath for that to start happening any time soon - but who knows...