Ferelden really isn't that large, given its political nature and rather vague boundaries. As a general region, sure, but not as a kingdom.
Tevinter is so small because of how much it had to retreat, though it does have holds to the west I believe. It may be 'small', but it likely is the most urban area of all of Thedas (perhaps rivaled by Val Royeux, but maybe not even there). Its dealings with other races and kingdoms, despite the Chantry's/Orlais'/Qunari's best efforts, keep it going. Magic will always find a place of power no matter how much others strive to regulate, eliminate, or nullify it. It isn't the huge reaching empire of old, but the size on the map belies how 'big' it may feel when we actually explore and learn about it.
Ferelden is pretty much done, except for either small field trips there or some hypothetical huge SP/MMO game that revisits it.
Free Marches is actually not done. We only had Kirkwall and surrounding area, but most of the Free Marches is untouched.
Orlais I'd say is, I dunno, half done? It was definitely explored, but there's also a lot that could be done with it later on, especially for city life.
Dragon Age could go on for several more games and still not explore all of major areas of the main map of Thedas, let alone the continent(s) across the sea (I'm wagering on just two others, one of which the Qunari came from). The unknown lands to the west and south of Thedas may be important at some point too, especially given the names of the locations near them.