First: base ATS isn't much smaller than base ETS2 (ATS: 992 000 km2 ETS2: around 1 000 000 km2) AND the latter reused the UKTS and German Truck Simulator maps.
Second: SCS map expansions aren't normally limited by boundaries but by area covered or number of hours of worked involved. I mean they haven't released just a Denmark DLC because it would be really small and if they would I guess they would have set its price accordingly. What I mean is while western states in the US are big and fit in one DLC, the more to the east the smaller they are.
Let's have a look at SCS DLC's to date:
- Going East: Western Poland, Slovakia and Northern Hungary - around 300 000 km2 (Release price: 10 €, lowest price: 1.49 €)
- Scandinavia: Denmark, Southern Norway and Sweden - around 250 000 km2 (Release price: 17.99 €, lowest price: 4.49 €)
- Vive la France: Eastern France - around 310 000 km2 (Release price: 17.99 €, lowest price: 12.05 €)
- (Expected) Italia: Southern Italy, Sicily?, Sardinia? - between 150 000 and 200 000 km2 (release price: 20 €? -source: Instant Gaming-)
As you can see, until now we haven't paid for a country but for a fixed amount of hours of work. The price went up since the first DLC, but also SCS has improved its quality and the amount of new models. And of course the number of hours of work and the team behind has grown. On the other hand, New Mexico DLC (according to Instant Gaming) is going to come out at 13 €, much lower than previous expansions. And that's after 1 year and a half out there with lots of free updates.
Third: The total area of ETS2 nowadays is around 2 100 000 km2. It'll grow up to 2 300 000 km2 with Italy. How much will it grow? Maybe Spain and Portugal? That would be another 582 000 km2. There are no other known plans to expand the map anyway. But until now there have been 4 expansions.
On the other hand, everybody is expecting/wishing that SCS build ALL the Contiguous States. Some even wish they build Southern Canada and Mexico and... Only the 48 contiguous states have an area of 7 653 004 km2, more than three times the current ETS2 map. It they keep going with their map expansion model, they need another 20 DLCs to have all the USA mapped (300 000 km2 each)... But there's a lot of work ahead and one should expect to be paid after all.