Some modifying thoughts ...
you are increasing the bending moment across the wing spar by increasing the weight inside the the wing spar attachment points
As, I think, Genghis made the point earlier, the wing root may not be the critical point along the wing. However, the general story is OK in that somewhere along the wing will be a structural consideration which limits MZFW. That can be modified, as with the L188s mentioned above but not without a quid pro quo somewhere along the way.
As I was once told, here are the limiting factors for the maximum weights:
A bit simplistic but a starting point for discussion, nonetheless.
Watch this Youtube video to see why you want to avoid straining the wing roots.
We need to keep in mind that there are, at least, two considerations at play with structural strength -
(a) instantaneous loading .. ie, put too much load on something and it will break
(b) fatigue loading .. much lower, small overloads, repeated over a lengthy period, progressively lead to cracking which reduces the capability of the structure to carry the loads presumed OK ref (a). If fatigue loads significantly exceed the design studies, eventually the cracking and section area reductions will lead to a failure at a much lower than expected load .. and much earlier than the OEM expected for the presumed service life.
For example, I own a heavy off-road caravan which Type is often subjected to a pretty hard life out bush. A number of examples have had stub axles fail at ridiculously low instantaneous loads due to significant fatigue cracking. Fix, in a manner similar to the aeroplane problem.. rework the design of the stub axle to improve the fatigue properties and ultimate load capability. That way, the owners can keep playing with a much reduced worry regarding the possibility of being embarrassed a long way from anywhere.