Question

Forces that act on an object but are not equal in size

Answer 1

Forces that are equal in size but opposite in direction and do not cause a change in an object's movement are called balanced forces.

forces that aren't equal in size and do cause a change in movement (what it seems like you're asking for) are called UNBALANCED FORCES

so answer (in case that wasn't clear, as I'm tired) : unbalanced forces