Simons is building a 92,000 square foot store in downtown Calgary (actually, I think it will be opening soon), over four levels in an old historic building. The Rideau Centre store is 104,000 square feet over two levels, a portion of which is in next to a rebuilt historic store facade (Ogilvy's).
If the footprint here is 12,000 square feet, then a four-storey store would only yield 48,000 square feet. Also, quite notably, the store would not have any connection to the main mall (quite unlike the Calgary and Ottawa stores).
Even if a potential Simons spread into the rooftop addition, it would still be significantly smaller than the Calgary store, and quite inefficiently spread over 6 or so small storeys. Lots of escalators. And still likely no interior connection to the main mall.
ETA: Just to be more accurate.