If you're comparing file sizes, it has no bearing to anything in Unity - the actual sizes will be the optimized asset sizes, which is undoubtedly going to be higher than your source PNG size.
You could try the compressed formats. 16 bit dithered will half the asset size, and not look completely horrible. If you have loads of empty space in your textures, you could dicing to reduce file size. Ultimately what you need to do depends very much on your assets and what your specific requirements are.