It's funny when an environment changes between films, isn't it??

Perhaps I'm over-analysing the issue, but given the effort that went into maintaining continuity elsewhere in the film, I found this to be a little odd!

The miniatures used in the production of the movies have been taken apart and reconstructed more than once in the course of filming, and the use of digital matt paintings complicates things a little further. Rivendell's layout, it seems, is not necessarily entirely consistent from scene to scene. Given Elven magic played a part in its founding, that is not surprising, nor inappropriate, but it poses a challenge for us as we seek to recreate it in earthly polystone! A certain degree of compromise has had to be found in order to replicate the Elven outpost so that it most closely matches the most iconic shots in the trilogy, but there's some ambiguity involved. We've had to reason that perhaps what we see over the shoulders of the Fellowship members in some shots represents some other parts of Imladris not discernible in the wide shots?

After careful deliberation and comparing all of the different sources, including film stills, the shooting miniatures, conceptual art and after discussions with designer Alan Lee, we have settled on creating a rendering of Rivendell that reflects what we see in the establishing wide shots.

The exact geography of Rivendell was never explicitly defined during The Lord of the Rings, but this times around we have established an area of Rivendell quite accurately, giving sets and digital work a common grounding. We approach it from a new angle, coming down a different face of the valley, so the entrance is totally new.

And since it was never really defined, it doesn't really matter that it's different again in The Hobbit.

i would agree there are times that the space feels a tad too small in AUJ, especially the dinner scene. but that's just as much an issue in LotR.