Molson Stadium has never shrunk, only gotten higher capacity, and still has plans made to expand. It used to house only around 18k, now it’s up to 22, and plans are to adds stands and skyboxes to raise it up to near 26k.
The Als Olympic stadium was used, but houses over 50k people, and it never sold out. They had 15-18k people in that huge stadium every game, and when they were forced to play at Molson Stadium at Mcgill, the fans were packed in together, and it made for a much more fun/entertaining game for the fans. So they never went back to the Big O.
However, due to thier success, and growth of fan base, they play playoff games at the Big O because it can sell out, and it’s covered late in the season.
2002 Grey Cup was at the Big O too, I was there watching Calgary beat on Winnipeg, then I went to Winnie’s on Crescent and 5-6 of the Blue Bombers were there drinking away the loss.