From the following two links for Airbus A330-300, the length of "lower deck bulk cargo compartment" has two different values.
a) Air China lists the value as 511 cm
b) Airbus (see pg. 112) lists the value as 404 cm
Some airlines have different specs. I can't comment on the examples you've given but one example I have encountered is on the A340-300.
The airline I worked for had conventional LD3 loading where the cans were loaded in pairs but for a while we had an A340 leased in from Swiss or Sabena (cant really remember which as it was more than 10 years ago) which had an additional LD3 position on the centreline of the aircraft in the space that would usually be available for bulk loading.
On thing which could affect the size of the bulk compartment is the location and number of toilets on the passenger deck. The plumbing usually lives under the floor.
Note the airbus site mentions that dimensions are approximate.