There was some discussion about this back on the Mystara Message Board at the Wizards forums when Thorf was working on his Darokin map. I believe there may also be a little bit of discussion about that in the Darokin map thread
along with the map he made which does show what the consensus Borderland territories were.
I can't check that map at work, but I think by and large the Borderlands were mapped as you note, with the assumption that the hexes shaded in on the 24 mile map on p. 11 were the actual borders (some of them might have been broken up; again, I don't recall).
That said, I tend to agree with you that those borders should probably not be representative of the size of individual Borderlands, and frankly they should probably not be entirely contiguous; there is likely a lot of wilderness and/or contested territory between individual holdings. Module CM9: Legacy of Blood- from where the Borderlands concept originated, IIRC, shows one Borderland dominion that fills a typical 24 mile hex (the base for the Dominion rules) that sits on the southern border of Alfheim.
(Note that the location of that borderland- Fenhold, IIRC- from the module conflicts with the later map of Darokin in that the 24 mile hex it occupies doesn't fit with the location of the town of Dolos. Personally, I like to keep it there, and just bump Dolos slightly further from the hex it currently sits in- attributing it to the elements of scale, but that the general consensus on that front was that Dolos should stay where it is, and Fenhold should be bumped instead.)
In short, there wasn't much specifically developed in canon. There have been some "fanon" things, such as the map above, as well as a few scattered entries in almanacs and things that you might find on the Vaults. If I remember later, when I can check the Vaults, I'll post the ones I recall. I know one was in the southwestern region near the Forts and the Tiger Clan territory (Tenobar region). I can't recall if it was James Mishler or Marco Dalmonte that did the writeup (or someone else; possibly multiple people).