Whereas the mapping procedures (sgmap and gmap) preserve the x/y proportions, sgplot will 'stretch' the graph to fit the available area. In your case, you could move your legend to the right-hand side, which would allow the graph axes to get bigger in the y-direction (and it would also 'squeeze' the graph in the x-direction). Or you could change the size/proportions of your output area.
In short, you have to control the proportions of your sgplot, such that the map will 'look right'.