Both Super Junior and Exo are from SM Entertainment. They both also have a large number of members comparing 13 to 12. Both groups are like brothers. But many people tend to call Exo the younger Suju. People just see them as brothers. 由 the way, I've never heard of Suju offending Exo before.
Well, it because they're come from same company and have similar number of members. And maybe because both of them achieved such a great achievement and record, like Exo with million album seller and SJ with being 1st for 100 weeks in Taiwan, people just likes to compare them.