I would certainly say that this is a route specific issue. As a daylight sector people eat and drink probably 10 times more than on LAX-SYD (or even on the return JNB-SYD which are overnight flights) 300 people all drinking soft drinks and beverages, 50 cans or so of coke zero don't go very far, and there is only so much capacity on an aircraft (and this flight already has additional uplift of drinks due to the nature of the flight)
FWIW in First and Business the catering loads have been the same in relation to the number of passengers for as long as I have been at Qantas (8 years). There have been no reductions. It really does just depend on what passengers feel like.
For example some flights no one has a steak sandwich, and others they are all gone! My last flight no one had the lobster, the one before it 2 did, the one before that we had to apologise to someone as there were to many orders for it.
Catering loads are monitored for every flight. If there is an obvious trend (so multiple flights with lots of people missing out), they adjust the catering accordingly. A recent example of this is that during winter, for Y class breakfast, more people have hot breakfast so they increased it to 80% hot 20% cereal. When summer starts, it will revert to 70% hot 30% cold.