A lot of the capacities of CFL stadiums are legacies from when those teams would draw more than they do today. The Argos are lucky to draw over half of BMO's capacity, the Stampeders will be playing in a smaller stadium in whatever replaces McMahon.
For Mississauga and other growing cities, it should be advised to start small and build their way up. There's no sense starting out at 20K capacities when there are hardly any franchises that garner that sort of crowd. Start small, gauge demand, and build from there.
It's unwise to glimpse at population numbers and assume all cities are built equally. Just because a city has X pop. doesn't mean they're all sports fans, and certainly doesn't mean they're all similar kinds of sports fans. How many hockey teams have failed in suburban Toronto in the past twenty years?
MLS has provided the blueprint for what Mississauga is looking for. DC, St Louis, Nashville among others have built urban stadiums in the past decade with varying degrees of success. Sauga should be looking to them for smaller-scale solutions. I think Louisville City's stadium may actually be what the goal should be in the medium-term.