A 3KB PNG which converts into a 3MB XNB is either an image with very few repetitive colors or has a lot of empty unused space.
The content pipeline just transforms the images you have in something which is very easy for the target computer to handle, in example, a raw texture or DXT texture. However, easy means efficient in time, but not always in space wasted.
If space used is really a concern, you can just store the PNG without going through the content pipeline and load it with a different loading function which can load raw PNGs. The PNGs will load slower (can't say by what amount) but will take very little space.