You are probably seeing a lot more of these small girls make an appearance because, as the sport has progressed, so has the tumbling and stunting abilities of these young kids. There weren't as many 12-year-olds on senior Worlds eligible teams in years past because there weren’t that many 12-year-olds who could tumble and stunt at that level.
Things are different now. As the sport becomes more competitive and grows, these young ones are acquiring elite tumbling and stunting skills that rightfully qualify them to be placed on a senior 5 team when they turn 12. I'm sure there are some itty bitty girls who move up without the skills, but any video or performance I've watched lately shows a good many of these little girls banging out doubles, trick tumbling, standing, and having amazing flexibility. If she's light and easy to put up, then that's just another bonus. I don't think you can blame the coaches for taking advantage of that or the athletes for accepting the positions. You have to either change the age/weight limit, or accept that everyone is following the rules.
Going forward with the new push for more guy/girl stunts, it may end up being more about the boys, than the girls. It would be great to have 18, muscled-up, college size guys on your allstar team, but we all know that's hard to make happen. I don't think most programs will have the luxury of replacing the smaller boys with bigger guys who can stunt. Most gyms will try to use any boy they can get on the team and pray they have enough to even go medium or large. A lot of these boys are going to be young and small themselves, so they're going to need flyers they can hold up. Until this sport reaches a point where coaches have a surplus of guys to pick and choose from, there may be a need for smaller girls to help some of these smaller boys out as we transition to more coed stunting.