Teams do carry larger rosters out here, however 20+ is rare. It is also fairly common for SoCal teams to pick up some players for Nats, so you really need to look at their website to see their normal roster. Corona Angels is the only org I know of that regularly carries 20+, but they also know how to manage them.Younger age groups 10-13u - 11 girls. 14u & up - 12-14 girls. Jobs, boyfriends, school activities, and type of schedule are all factors. If you listened to the podcast, western teams carry almost a full roster (20+) girls. I'm seeing more teams in Ohio carry 12 and I think we will see that number rise in the coming years as coaches learn how to manage the larger size rosters.
Here is a 2 minute video where a couple of tOSU recruits talk about how it pushes them - Corona Angels' Taylor White and Ashley Goodwin.
The roster on Marty's website shows 19 players (all committed) and his PGF roster has 20. Comparing the rosters, his 4th pitcher left and he picked up an IF and an OF. The PGF stats for 2 bracket games over 2 days show 17 players have batted and 2 of his 3 pitchers have pitched (click here).
Roster size really comes down to the ability of the players and the coach to manage it. Coaches shouldn't simply add bodies to have a larger roster. They should only take additional players that are capable of playing at his team's level and will push the other players. They also shouldn't take additional players if they're not comfortable setting up player rotations.