The A330F holds 22 AMJ/M1N (96x125x96) containers, or 20 AAD/A1 (88x125x96) containers and 3 AMJ/M1Ns; it also has 8 belly positions.

The 767-300F holds 24 AAD/A1 containers and has 7 belly positions.

Main deck volume is a wash, though the extra belly container of the A330 gives it an edge in total volume. The A330F also has better range/payload flexibility.

An A330-300F would allow 26 AMJ/A1N, getting it close to a MD11 in total volume...but with nowhere near the range.