Actually the resolution is higher than a 6mp camera. It is much closer to 8MP as the dpreview results will protray. At any rate I do get much less resolution from an image developed in lightroom as compared to ACR and it is frustrating not being able to take advantage of it.
I'm not sure whether you are confirming what the others have assumed here or not, but if you are, then the problem does not lie in developing nor exporting, but rather in importing
This seems rather strange, since Lightroom uses or at least used the same raw engine as Photoshop/ACR.
When you view your raw image in the library, what resolution does it claim that your image has, in number of pixels width x height (see example below)?