Why does stepsize in stream2 only adjust the x-coordinate stepsize?

I'm using stream2 to get streamline coordinates. I want those coordinates to have the same discretization as my velocity data, so I changed the stepsize to one. This, however, seems to only affect the stepsize in x, but not y, so I can't use the streamline coordinates to index my velocity field. I realize that coarsening the stepsize in both directions will yield ugly streamlines, but what else is stepsize good for?