I downloaded an mp3 today that is:
Size: 17865122 bytes
Header found at: 26624 bytes
Length: 445 seconds
MPEG 1.0 layer 3
320kbit, 17086 frames
44100Hz Joint Stereo
CRCs: No
Copyrighted: No
Original: No

then i have another mp3 a little bit smaller in size but the overall size differance is 10 MB more.. why ?

Size: 7843968 bytes
Header found at: 708 bytes
Length: 392 seconds
MPEG 1.0 layer 3
160kbit, 15025 frames
44100Hz Joint Stereo
CRCs: No
Copyrighted: No
Original: No
Emphasis: None