Hello, Witcher3 composer here. You might wanna check your sources - Witcher 3 soundtrack is 6 hrs long. And if you'd count all the layers, variation segments etc. (so all adaptive/dynamic stuff that is happening under the hood) the number could be even higher.
The fact that TW3 Official Soundtrack compilation is only 70-something minutes long is because we were limited to a single audio CD per SKU. So we decided to pick the best (in our opinion) tracks that would show overall tone and feeling of the game.
Skyrim's Official Deluxe Soundtrack (that super limited one, sold by Jeremy Soule himself; I've got one of these, with Soule's autograph btw) was published as 4CD SKU, with 3 CD's packed with music tracks and additional fourth with sound/musical soundscapes, called "Skyrim Atmospheres". It's even featured as a single track, with duration of 40-something minutes.
I'd like to remind you that on some markets (I don't know whether it was worldwide or not) standard edition of Skyrim's sountrack had only 4-5 pieces. Does it mean Skyrim soundtrack had only 10-20 minutes...? Oh, and if you bought collectors edition you didn't even get the soundtrack (got myself one of these, had to buy soundtrack separately).
It's impossible to say which soundtrack is longer (you'd had to extract Skyrim's game files and compare them with Witcher3's sources) and really - there's absolutely no point of arguing about that. The only thing common to Skyrim and Witcher3 is general genre - RPG. All other things are different: setting, main protagonist, approach to open world and storyline(s), combat system, and - also - music direction and it's implementation. Try to compare stool with a couch. You sit on both and that's it when it comes to similarities. Good luck with that.
And by the way - I encourage you to play the Wild Hunt and give it a try. You might even find out there's a lot of ambience/atmospheric music as well. Literally hours of it. And it's a really fun game