The short answer is: Yes, Apple Car Play is a Data Hog!
CarPlay uses the Maps app in your iPhone, as navigation system, and Maps (as opposed to Google maps, or any other navigation app on the marked) has no option to download a fixed map for the app. In stead the app downloads small segments of a map showing you the area that you are currently located.
I don't know why Apple won't allow their users to download maps for the app, but guess that it could have something to do with the fact, that they are getting their maps from Tom Tom and therefore could be some legal/licensing issues prohibiting the use of downloaded fixed maps for the Maps app.
It is off cause extremely unsatisfying, from a users point of view, as you are forced to stream a lot of data, whenever you use Apple CarPlay for navigation, and it also renders other issues such as missing maps, and thereby missing navigation, in areas with low cellphone data coverage.
I have addressed the issue to Apple, a number of times, but it seams that they don't relay see the problem or perhaps the just don't care?
A solution to the problem could be that Apple allows other navigation vendors to use their systems on CarPlay, but as is, the CarPlay system is exclusively for Apples own navigation system and they wont let other vendors in on the marked.
Hope this clarify your concern.