You've already been given the answer but I will try and expand the explanation a bit. JPG is a lossy compression file format for images. The camera also can shoot in RAW format, which uses lossless compression.
In lossless compression the file rearranges the information so that any patterns that are repeated in the image are stored once instead of 2/3/4/??? times. RAW data is also stored in 16 bits per channel per pixel, as your ADC is 14 bit.
In a lossy compression scheme you actually throw out some of the data, usually by converting the image to HSL format and storing less luminance data, for instance. JPG is an 8 bits per channel format, so it's much less data than the RAW information.
In short, the more patterns in your image, or closely related color values, the smaller the file size, so an image with a bunch of evenly lit solid colors will result in small files, while a photo of something high frequency (lots of rapid changes in the image) will result in a large file size. Noise in the image counts as high frequency data, and does not compress well.