What higher resolution was it shot at?
MB is not a direct reflection of "resolution". Why is 21 MB a problem?
When you import the photo into Lightroom, there are no changes made to "resolution" or to MB. Whatever you are seeing with respect to "resolution" and MB are exactly what the camera produces.
How do you believe that you shot at a higher resolution? But there is no relationship between resolution and file size.
In the Library module, select an Image and press the "e" (loupe) key to view a single image.
Then press the "i" key until you see some information as shown here:
NOTE. You may need to press the "i" key more than once. It cycles through 3 options.
Let us know what the pixels dimensions are. That is the resolution of your file. This happens to be a 5D MKII file, full resolution, un-cropped. As you know the 5D MkII can shoot 3 different resolutions.
The "weight" of the file in the finder is 23MB. Files will vary around the 21-23MB mark depending on content.
Let us know if this helps