The symbol or bar that precedes a legends is larger when the text contains a Greek character have (preceding the text) than it is when the labels contain no Greek characters. 

As you noted, the size of the precding bar or symbol in the legend relates to the "size" of the text. Somehow Greek letters confuse Prism, and the bar is bigger.

Here is a way to work around this bug. Put a greek letter on all of the legends. That way, the size of the preceding bar or symbol will be equal. For legends where the Greek doesn't make sense, select that Greek letter and make it white (or whatever your page background is) so it is invisible. Do this from Change... selected text. Prism will "see" the Greek and make the bar larger, but you won't see the Greek since its color matches the background.