In the Bridge Designer, your design is judged to be successful if it passes the load test–if the truck is able to drive completely across the span without causing a collapse. Thus, to be successful, your structure must have adequate strength. In evaluating your design, WPBD does not consider how much your bridge bends.
Actual bridge designers do, in fact, consider whether or not the deflections of the structure are too large; however, large deflections are not a safety concern. Deflections are considered in structural design only because excessive movement of a structure tends to make people feel uncomfortable–no matter how strong the structure actually it is.
In any case, your design probably does not bend too much. It only looks that way! In the Bridge Designer Load Test Animation, the displacements are exaggerated by a factor of 10. The purpose of this exaggeration is to illustrate how tension and compression in individual structural members cause the entire structure to bend. If you would like to see how much your bridge would actually bend if it were built, click the View menu in the Bridge Designer. Then select “Load Test Display Options…” and uncheck “Show Exaggerated Displacements.”