they've been saying this:

Jobs are specialized party geared alternatives to their counter-part of classes. I can look it up for you but I am pretty certain that Yoshi-P himself stated that Jobs are more specialized meaning which they get access to new special abilities but are unable to use other classes abilities in turn.

Hence Jobs = large parties
Classes = smaller parties/solo

Like I said I can look it up for you, but it will have to wait till tomorrow when I get some more free time on my hands.

Edit: Just to clarify, a job will not lose out on its class's ability. IE: DRG will be able to use all the LNC abilities, since its a specialization of that class, however, DRG will probably not be able to use all or any of the THM/CNJ abilities while LNC will be able to.