A quick bit of info on why and how the numbers vary.
What Unity shows you in the size of the asset in memory, not on disk. An uncompressed texture HAS to be uncompressed in memory.
A 1024x1024 32 bit RGBA image with mipmaps - 1024x1024x4 x 1.33333333 = 5.3MB
Note: you can turn off mip maps if you don't need them and save the 1.333333 bringing the texture down to 4MB.
The file on disk is a PNG. A png uses zlib compression and some optional transforms per scanline to keep the size down - its lossless compression is useless outside disk storage. The size of the PNG has no bearing to the size of an image, eg. empty images will compress better than images full of content.
Unity performs some form of compression when building the asset (you can check this by looking at the asset size in the editor output log, and comparing with the final assets), and performs REALLY aggressive compression when saving a webplayer. I don't believe there is a way to chose what kind of compression is performed. Can't remember off hand what kind of compression is performed and where, though