Captured image is bigger than the acquired RAM

Hi folks,

Could someone explain to me why the image file of the memory captured in raw format is bigger than the RAM memory itself?

I have 16GB of RAM and made memory collections using FTK Imager or RAM Capturer, in both programs the image generated has 18,866,176 KB.

Why the file size is not 16,777,216 KB (16 * 1024 * 1024) KB?

Thanks in advance.