PNG file is a lossless file format it uses LZW compression when written in physical storage that’s why it’s small in size compare when compiled to XNB.
PNG file compiled to XNB is much larger in size because its UNCOMPRESSED when written to disk for the purpose of NOT decompressing it on loading time meaning it’s much FASTER.
Normally our physical storage is much larger than our memory I couldn’t care less the size on disk as long it’s much faster in loading time.
EDIT : Other than that, we have an option to select Color key, Generate mip maps, make it square or resize to power of two and other fancy things the pipeline tool has to offer : ))
Cheers ^_^Y