Resize Image but retain image size

When I resize the image to 2000 pixes (72DPI) in photoshop it tells me the image is 7.8mp - great!  But then when I go to save, the jpg file is only 2mp.  Am I missing something?

By @mellieevans

This is jpeg compression. That's not a reduction in resolution, but an encoding algorithm for the data that results in a much smaller file size for storage and transfer.

What you see in Photoshop the "native" image size for a flat image - the pixel size x channels x bit depth. The jpeg algorithm compresses that further. Unfortunately, that compression is non-reversible, so a jpeg can not be restored to the full original quality. But for most practical purposes it's good enough. A jpeg can be down to 2-5% of full native file size.

I think you might be mixing up MP (megapixels) and MB (megabytes). But, so might be the rules, if they really say "and compressed to 8MP". Please double check the wording. Of course competition organisers can get confused as easily as anyone else.

If the rules, and what you say, seem correct, please show us the screen shot of where you find the file is "only 2mp". 

Also, when someone gives a compressed size as 8MB (not MP) you should treat this as a maximum or minimum or guide (according to what they say), since nobody can get an exact size in MB. You can use JPEG quality settings to influence MB (not MP).