ArcMap 10.2 legend symbol sizes wrong

I have a simple map with graduated size circles representing greater quantity of numerical field in a table.
When I add a legend, the size of the corresponding circles is about twice the size of what they should be.

I do not have a reference scale set.

If I increases the size of the symbol in the map, the legend will increase the size of the corresponding symbol, always about twice the size as what I set in the map.

Any idea what I am doing wrong here?