LOL, where did you come up with the notion that Miami/South FL and Jacksonville have topped out?
Jacksonville is one of the biggest cities in size in the country so even at 800,000 it's small. The population of Dade County on the other hand is at 2.5 million, and that of Palm Beach at around 1.3 million. I don't see them slowing down anytime soon.
And you forgot about Tampa-St. Pete which are growing nicely.
Since the Tampa-St. Pete radio market includes two counties that are not too close to downtown Tampa (Pasco and Hernando) I see no reason for Arbitron to scale back the size of the Orlando geographic market. After all, Tampa is already a Top 20 market.
Furthermore, the Orlando TV market is the one that covers too big of a geographic area as it includes Daytona and Melbourne as well.
The biggest problem in metro Orlando is that most of the fast roads are toll roads and there's only one interstate. If there was an I-44 and an I-84 looping around parts of Orlando alongside I-4, rather then the 408 and the 417 then traffic would move faster. Miami has I-195 and I-395, and parts of 826 are free.
Tampa has I-4, I-75, and I-275, along with some good toll roads.
So if any market stands to slow down it's ORLANDO due to a lack of proper interstate planning and funding.