Though counter-intuitive, this is actually works against an airport west of Pearson. A western airport splits the west of the GTA with Pearson, leaving all the Eastern GTA and a good chunk of the Western GTA in the Pearson catchment area. If you built an airport in Hamilton or KW, the entire GTA east of, I dunno, the Oakville-Burlington border would still drain into Pearson. That's the bulk of the region's population.
Pickering's catchment area would have been smaller than Pearson's since the GTA is western-heavy, but bigger than Hamilton or KW.