Truss optimization using Fmincon

I am a beginner in using matlab so here is my question. I am performing structural optimization on trusses considering overall and local buckling with weight as the objective function. My code is concerned with box and rectangular sections so when the buckling length in x and y is the same I expect that the result from the optimization process will be a box section but this is not the case when the applied load increases. I figured out that if I mutiply the objective function, the optimized members become almost box sections. So my question is How can I determine what makes the code sensitive to the value of the objective function? Thanks in advance