winter loads also tend to be lower
FR24 shows A332 on those MU flights most of this year
Im sure you can even run a 230K A333, just not with a good load and with how Air Asia are likely to configure their planes, unless they start going more premium, even a A339 is going to be stretching it.
Throw in more facts before jumping to your conclusion. Far more airlines are running A333 flights of that stage length than you thought.
Now, LAX-NRT is a 10:30 flight in the summer, 12hr flight in the winter. LAX-KIX is 11:10 in the summer, 12:40 in the winter. I'll throw in some 12h A333 flights just to give you some extreme example:
QF107/CA174 SYD-PEK (and for QF107 that replaces some A332 frequency).
BF705/1 RUN-ORY (heck a long flight for a sardine can)
CA845 PEK-BCN (when it swaps to A333 in November)
Now, I'm not saying every single of these flights go out full or without penalties, or if D7 throws the same plane onto the same route it won't face penalty (partially because of the reason you said). But the way they schedule A333, particular those new planes show that at least in their config they don't have much payload concern if at all. All of them have more capable planes if needed.