Forget about "resize to fit megapixels" option, that simply doesn't work in LR. Probably a bug...
I spoke too soon. What I meant is that it doesn't work as most people expect. People mostly use it hoping to arrive to a specific file size in MBs - Megabytes. As in: "do not send files larger than 50 MB," for instance.
Lightroom's "Resize to fit" function is, however, designed to deliver specified Megapixels, not Megabytes. Whether the output file is jpeg or tiff, 8 bit or 16 bit, Megapixels are always going to be the same, as they represent file's dimensions. What is going to change are Megabytes, depending on subject matter, compression algorithm and desired bits.
For instance, in the attached example, an originally 8-Mpx file from Canon 20D (3504 x 2336 pixels) is enlarged to 20 Megapixels (5477 x 3651 px).
The jpeg Megabytes size is only 5 MB, due to 8-bit file and compression, while tiff files are either 120 MB (16-bit file) or 60 MB (8-bit file).
How does a 20-Megapixel file become a 60 Megabytes one? Color files have 3 RGB layers, red, green & blue, and each layer is 20 MB, as requested. Having a 16-bit file doubles the output in Megabytes to 120.