Reduced file sizes in Lightroom Classic

You probably set the camera to record a reduced crop size (1.2 or DX) or enabled a reduced size raw file instead of full size raw. The D750 has the following resolutions you can set according to https://www.dpreview.com/products/nikon/slrs/nikon_d750/specifications  :

Setting the camera up with any of these will reduce the MP size of the recorded raw to these pixel dimensions. So in FX, you can have 24.1 MP, 13.6, and 6MP. in 1.2 crop, 16.7 or 9.4MP. In DX you can have 10.3, 5.8, or 2.6 MP. Apparently you likely have the 1.2x crop selected in camera. 

It is also possible to set the camera to a 4:5 or 4x3 crop which also reduces the image size in the raw. Also when you record images while in video mode, it records a raw with a 16:9 aspect ratio, again reducing the image size. So check your camera! settings 

Your Nikon D750 takes 24 MP (that's megapixel) files. They may show up as 16.1 MB (that's megabytes). Megapixels and megabytes do not have to be (and usually are not) the same. There is nothing wrong with this. Do not interpret this as a problem.