It's a coincidence that the Sony and Canon megapixel sizes are so similar to the megabyte sizes. There is not normally a correlation between them. The file size in MB can can be different numbers for the same number of pixels, depending on what type of data is in the file and what type of compression is applied.
When taking DNG photos, the file size can be quite different depending on the camera mode used when the photo was captured. For example, taking DNG photos with the Lightroom camera on my phone's 12 megapixel camera, a DNG photo captured with Auto mode is 14.3MB, while a DNG photo captured with HDR mode is 40.5MB because it’s a merge of multiple exposures.
Do you get different DNG file sizes on the Nexus phone when you use different camera modes?