I get what you are saying, but looking at Iowa City's growth, it says that is in Tiffin which would put those kids in the new high school they just opened a couple of years ago. IC West lost athletes to new high school also.
Maybe it's just me, but I can remember when the large school classification was owned by the Eastern Iowa schools, along with Dowling, which choked in the end and Valley.
Last year SEP had two of the better football players in the country, the DB that went to EIU and the tackle that went to Alabama, we also got a couple of their players. When was the last time eastern Iowa had a highly ranked player come out of high school from that part of the state. It's been a while.
Growth in Tiffin doesn’t put those kids in the Iowa City district, it puts them in Clear Creek Amana, which would be along the lines of the Dallas Center Grimes/North Polk/Bondurant-Farrar types of districts; fast-growing, but not up to 5A.
Iowa City did divide their students up some by opening Liberty, which mostly cannibalized West. Still, Ankeny and Waukee have done the same, without that much of a hit to their football successes.
I don’t have the answer to why the DSM suburban schools have dominated since 2010; there’s some combination of coaching, investment, and population trends at play, but I think it’s population that’s the biggest factor. In the east, I think the districts benefiting from the move away from rural areas and inner cities are still 3A and 4A; in Central Iowa, where there were more people to start with, 5A schools like Southeast Polk and Ankeny and Waukee are reaping those population trend benefits instead. And of course, the built-in benefit of being a large private school like Dowling in the population center that is West Des Moines gives them a boost.
(That said, even 4A is starting to see a power shift to central Iowa, with schools like Bondurant-Farrar and North Polk and ADM and even smaller districts like Van Meter are getting pretty powerful.)