Large load impedance will draw a small load current and so loading of the source is small. (light load). A small load impedance will draw a large load current from the source. (heavy load).

The effect of a load on a transmitting interface. The effect of connecting one device to another. The effect of a voltmeter upon the circuit being measured that results in an inaccurate measurement. L [..]

Connection of a meter (an ammeter in series or a voltmeter in shunt) to a circuit to make a measurement alters the original circuit in that it draws some energy from the circuit. The error caused is known as the loading effect.