Underrepresented can imply that the group can do more to represent themselves.
Meanwhile, the overrepresented majority is usually the group that possesses the power to influence which groups are majoritised (and subsequently receive power) and which groups are minoritised.
When using any term for a group, it’s important to contextualise it into people. So, for example, “people from the overrepresented majority are less likely to be affected by this change”
