Certainly in Australia, the answer is no. You may not reduce the category because you have a lower weight. I believe its the same for ICAO, but do not have the reference here.
An aircraft must fit into and be operated in accordance with the requirements
of only one category. An aircraft:
a. may not reduce category because of reduced operating
weight, but
b. must increase category when actual handling speeds are in excess
of those for category (based on Vat) detailed at sub-section
1.15.