Base64 uses 6 bits per byte to encode data, whereas binary uses 8 bits per byte. Also, there is a little padding overhead with Base64. Not all bits are used with Base64 because it was developed in the first place to encode binary data on systems that can only correctly process non-binary data.
Basic answer = gzipped base64 encoded files will be roughly comparable in file size to standard binary (jpg/png). Gzip'd binary files will have a smaller file size.