It doesn't matter what machine a program is written on. What matters is the code (and the compiler.)
The code haven't really changed though, besides adding some achievement, various minor stuff and bugfixes with the 1.6 patch (and soon more bugfixes with the 1.7 patch).
At heart Dawnguard was made by using the http://www.creationkit.com/ like any fanmade mod out there, and then 3DS MAX for new models and Photoshop for textures.
Unlike most games out there, the Bethesda games are pretty much only coded for general case. Basically, the Skyrim is sort of like Microsoft Word, and the master file is a document. The program has to handle the general case for everything -- terrain, buildings, statics, items, NPC's, etc. So the code remains the same. While other games have specific code to handle specific areas, they can pre-calculate lighting (even with weather & time of day changes), they can optimize paths and collision, etc.
So they been going with better and easier to develop content over render speed, which I think is a perfectly valid trade-off to make large handmade game worlds, and also proven to be a very GOOD thing for the Bethesda modding community.