I don't know for sure, but I seriously doubt that they are being converted to 8-bit files. The amount of reduction varies from one file type to another. If you have Photoshop, send one of those images from Lightroom to Photoshop and you should get an indication of what bit mode the file is in.
I converted all my NEF (nikon D700) 14Bit to DNG using Lightroom 4.
The file size of NEF is 22-26Mb
If your D700 NEFs are 22-26 MB in size, I'm guessing you are shooting in uncompressed format. DNG will compress the data while retaining bit depth, which in your case will result in a major space savings.
Yep, your guess is right, i'm shooting in 14bit uncompressed format.
So you think that DNG is mantaining 14bit depth but it's compressing the data? I hope with a lossless method I'm warried about loosing quality!
I tried opening files with photoshop (camera raw); the files was opened with 16bit
First, there is absolutely no need to shoot in uncompressed raw on the D700. Change the settings to "lossless compressed", and your raw files will only be about 14 MB each. You'll be able to fit more shots on each card, the images will download faster, and you'll save on disk space. The resulting images are mathematically identical to an uncompressed raw.
Yes, a DNG places the 12-bit or 14-bit data into a 16-bit container, so no data is discarded when you do a lossless conversion. It is only when you opt to use lossy compression or downsize the image when some data obviously needs to be thrown out.
Once the image is opened in Photoshop, it will not show whether the source was 12-bit or 14-bit. Photoshop documents exist as 8-bit, 16-bit or 32-bit documents. There are no in-between bit depths, since they operate in multiples of 8 (8 bits in a byte). So whether you open a 12-bit or a 14-bit NEF, they will appear in Photoshop as either 8-bit or 16-bit documents, depending on your settings.