Power steering heavy then light why vauxall corsa?

it depends on the age of the vehicle.

Older ones have hydraulic PAS. The fault could be in the pump, the Rack, a leak, or as simple as a low fluid level.

More recent Corsa's have an electric system (you will see a light on the dash "ESP" for Electric Power Steering when starting the engine.)

If you see this light when starting or driving, the fault is in the steering column or control unit, and is not uncommon.