You know, FCP and Big John (or AON, for that matter) are not really all that different in height. But Chicago looks so much taller than Toronto. I think that this is the case for three reasons:
One, the Chicago 'big three' (Sears, AON and John Hancock) are well separated from each other, forming the central pinnacles of three distinct concentrations, with each being considerably taller than the neighbouring highrises. By comparison, FCP is surrounded by a dense collection of highrises, some of which are almost as tall as it is itself. This reduces its visual impact from a distance, compared to the Chicago tallest buildings. If the Chicago big three had been as close to each other as the MINT towers are in Toronto, their impact on the skyline would have been reduced too.
Two, the CN Tower dwarfs every highrise close enough to be compared to it, including the entire CBD. FCP verges on being a supertall, but looks short in comparison. Put John Hancock next to the CN Tower, and it would not look nearly as impressive as it does now (in my opinion).
Three, Chicago is genuinely full of talls, many more than in Toronto. However, I saw Chicago back in the 1970s, before many of the talls that fill its downtown were built, and it still looked utterly amazing. I think that to create a maximally dynamic skyline, the tallest buildings should be well spread out, with lower buildings surrounding each tall and supertall, to fully display their height. (On the other hand, MINT-style crowding of the talls and supertalls into one very dense district makes for the most impressive street-level view, as opposed to distant skyline shots.)
Bill