there are a whole host of reasons for weight limits, and the manufacturer often doesn't say what these reasons are. Things like ability to hover (the rotor system can only put out so much thrust (lift) and the engine can only put out so much power, for example).
The landing gear's ability to absorb loads is another limit, as is the seat's crashworthiness.
If the Robbie could do more with safety, don't you think the manufacturer would try for it?