SMU has an undergraduate enrollment of just under 7000. SMU is in Texas, the BIG12 already has plenty of Texas teams including TCU in the DFW area.Why not take SMU?
I don't think bigger is better. I think there are 2 main criteria in expansion decisions, the first is long term viability and survival. I think that's what drove the BIG12 to take the 4 teams they took. If viability isn't a concern then the second reason comes into play, you try to add teams that you believe will enhance the per team value of the league.
It seems to me SMU would mostly just be an extra mouth to feed, and wouldn't make the BIG12 any more stable. Sure a very weak PAC may take SMU, but IMHO that's not a good enough reason to take such a small school.