Giga pixels uploaded inconsistent with file sizes on computer

Gigapixel - 1 billion pixels. 

https://en.wikipedia.org/wiki/Gigapixel_image

Gigabyte - 1 billion bytes. 

https://en.wikipedia.org/wiki/Gigabyte

pixel count vs data storage in bytes

Your camera is likely 20 megapixels. This means that one image is made up of 20 million pixels (.02 billion pixels). One image is probably roughly 10 megabytes of storage.