Eleven teams would give two no-plays in football; twelve gives you three no-plays, with or without divisions. Ten teams gives you one no-play, or an unbalanced schedule.
There are simply strengths and weaknesses with a conference no matter what number you reach. But some have more weaknesses than strengths.
For an 8 member league you have 7 games and play everyone. But the weakness is that you have rotating years of having 4 home conference games one year and then 3 the next.
For 9 you have a balance of 4 home games every year and everyone plays everyone. But the weakness is that you rotate a team that doesn't play every week -- less flexibility for the school as it must conform to the rotating conference idle team.
For 10, you are again at 8 games and can go 4 home games every year, but you rotate 1 team that you don't play through time (like the Pac 10). Everyone doesn't play everyone in a year, but you have a balance of home and road games and there is no need for a rotating idle team.
For 11, it has the most weaknesses. You have both the rotating idle game, everyone doesn't play everyone, and the math gets complicated in selecting the two teams each member doesn't play and rotating them. In the Big 10, each member also has a number of teams they play every year no matter what based on rivalries. The 11 team format has the most weaknesses and is the reason why most conferences don't want it -- ACC. Likewise, the 13 game format has similar problems as well with two divisions. The MAC went to 14 after a while and are trying to do that now with Temple entering and trying to get WKU to move up.
The 12 team league has more balance, but you don't play 3 teams every year and there is no need for a scheduled set of rotating idle teams. Also the math is simple and very easy to implement in determining which teams you play each year and which teams you don't play. For the Big 12, its simple. All 5 of the other intradivisional games and 1/2 (or 3) of the other divisions 6 teams for 2 years and then the other half of the other division for two years. Simple, balanced scheduling with no need of a rotating idled conference team and you have 4 conference home games every year.
Really, only 9, 10, and 12 are the ideal sized leagues. 8 is acceptable too, but has a weakness. 11 is the worst do to the math, the complications with rotating idled teams and lack of numeric balance of which teams each team plays every year and what teams you rotate that you don't play every year.
A far more interesting scenario would be if San Diego starts scholarship football:
The problem with U San Diego is they are located in a market where San Diego State exists, plus UCSD plays D II ball, plus the San Diego Chargers. They would need the revenue to support their program with scholarships in order to be competitive and make it worthwhile. They are not going to start a scholarship football program just because the Big Sky would offer them a league membership. They would've done it a long time ago if this was the case. What circumstances have changed since they went to non-scholarship 1-AA?
What I also would suspect is the possibility the conference is aware that there might be a forthcoming movement out of the conference-such as Sac State to the WAC for all sports.
These same issues were known when the Big Sky expanded the last time with UNC and chose not to take either the Dakotas at that time as well. 8 still allows them to be a conference, after Sac State leaves, if they leave.
Personally, I would try to shoot for the MVC if I were at NDSU or SDSU - the geography is not nearly as bad.
The MVC may be what is best for the Dakotas but it may not be best for the MVC. The MVC is made up of teams that are located in mid-major Midwestern markets (Omaha, Des Moines, Wichita, Springfield, Peoria, Evansville, Southern Illinois-Saint Louis, IL portion, NE Iowa (Waterloo/Cedar Rapids, etc). They are also somewhat tight geographically, particularly in the Illinois/IN area -- a little like the MAC. While Brookings is 240 miles from Omaha, 340 miles from Des Moines, 340 miles from Cedar Falls, and Fargo is 420 miles from Omaha, 530 miles from Des Moines and 530 miles from Cedar Falls, they are all considerable distance from the other 7 MVC members and they don't have the type of markets that meet with the league. On top of that, there is a big gap between how long the MVC teams have been playing Div 1 bball and their stature on the level of upper Midmajor -- A 10, CUSA, WAC level stature. They are not going to let in new move up teams.