Hello Luis,
I assume your network design problem formulation has some binary variables. Predicting how long it takes to solve a mixed integer linear programming problem sometimes can be really tricky. It does happen that a restricted problem is harder to solve than a larger version of the same problem.
A very much simplified explanation is the following: Restricting the problem means reducing the number of solutions. That means on the one hand, we have a smaller search space to look in. On the other hand it might mean that it is harder to find any feasible solution or it becomes harder to move from one feasible solution to the next.
In general I can say that larger problems are typically harder to solve, but there is plenty of very small problems that are really hard.
For linear problems (LP, i.e. no integer variables) there is a much more clear relation between size and time to solve. But even there small problems with a lot of degeneracy or difficult numerical properties might be much harder than much larger problems.
In terms of helping you to solve you instance faster, OPTMODEL allows you to set a lot of solver parameters that might speed up things for you. You can enter a technical support request here:
http://support.sas.com/ctx/supportform/createForm
and submit your data, I'll be glad to take a look at it and suggest some parameters that work well for your instance.
Philipp
