You are
always restricted by MZFM whether the flight is long or not, so put a small
amount of fuel in and you are going to be well short of the MTOM and hopefully
the MLM. If however you put a large amount of fuel in, MTOM and MLM is now a
factor.
Remember that fuel does not count as part of the traffic load hence it does not
affect MZFM. If traffic load remains the same and you add more fuel, you get
closer to the MTOM. Therefore to fill the tanks you may need to offload
traffic.