You will need to add a field to your population attribute table, using the % increase in population between two selected periods; keep the numbers as whole numbers for ease of use.
Then, from the Layer Properties dialog box, select Symbology......Categories.....Unique Values.....choose your new field in the 'Value Field' menu, then click 'Add All Values'. From here, you can select your own symbols for each increase quanty.
Or, for more advanced symbology, choose the 'Quantities' instead of 'Categories' in the Symbology tab, and let ArcMap determine the proportional symbol size or graduated color or symbol category, still based on your new field.
Bill; GIS Analyst