There is an error in how metric tire sizing works. The second number is the % of the section width (first number) that the hight of the sidewall is. In the example given, a 75%, the sidewalls are 75% of the section width. It isn't 75% sidewall and 25% tread. On lower profile tires, if the section width remains the same, the width increase will not be very great, so a 50 series tire with the same section width will be considerably shorter, but the width will be almost the same, perhaps within a 1/2" of the taller tire's dimensions. If properly plus sized to keep the same diameter, then the width will be increased greatly.

Bill Burmeister