I don’t think the comparison is entirely fair and perhaps a case of apples vs oranges but it is one that I’ve heard. Regardless, french frys have got to be one of the most unhealthy sides in Western cuisine. First, as a simple sugar, the potato is an icon of high glycemic index for those who maintain a lower carb diet. The starch in potatoes is rapidly digested and hits the blood sugar about as fast as sugar (my experience).
So, take something akin to a piece of candy and deep fry it in grease and pour salt on top (and/or cheese) and you get the American french fry. High carb, high fat and high sodium.
All that said, there are variants of fries that are better than others and many fast food restaurants have taken steps to reduce the amount of saturated fat while maintaining high standards regarding the purity of the cooking oil. All the info you need is here.