A big empty ship is cheaper than a tightly packed smaller one, but it can and will be filled with gear the ADF already has, will be getting, or could easily get. The larger crew isn't an issue either as the navy has a stack of sailors who would love to go to sea but can't because they don't have the required qualifications and or experience to fill the available billets. Sometimes its because there are not sufficient qualified supervisors or mentors to get these sailors up to speed. This is where bigger ships are useful, they can employ more trainees and ensure they have the supervision and get the experience they need and then can step, fully qualified into a skimmer.
As I stated before big empty ships with flat decks, hangers and maintenance shops are very useful for supporting deployed rotorcraft from other ships, even carrying spare or alternative aircraft for other ships. Then there are UAVs and UCAVs, as well as other arising technologies, such as 3D printing of components and consumables from raw materials on board, this is something the USN is seriously looking at for their carriers, LHDs and LHAs.
The counter argument is they are big, they look expensive, therefore we can't afford them.