Voltage drop in wire is determined by the "gauge" of the wire, which is the thickness of it. A 12 gauge wire is thicker than a 14 gauge wire. The 14 gauge wire will have more voltage drop caused by the current passing through it because it is smaller and has more resistance.
Voltage drop will also be affected by the amount of voltage on a wire. It would be incorrect to assume that voltage drop is a percentage based on the length of the wire. The wire gauge affects the resistance of the wire, but assuming the same size (gauge) of wire, a higher voltage will have a greater force behind it than a lower voltage so essentially the resistance of the wire doesn't affect higher voltage as much as it does lower voltages.
If you were to compare electrical wiring with plumbing, voltage would be equivalent to water pressure, and amps would be equivalent to flow, or gallons per hour.
Additional Comments and Answer for European Electrical Installations
The questioner probably really means "Why is there less voltage drop for a cable operating at 240 V, compared to the same cable operating at 120 V".
If this is the case, then the answer is simple. For a load of any given power, the higher the voltage, the lower the resulting current. For example, let's suppose we have a 1000-W load operating at 230 V -this load will draw a current of 4.35 A. The same load operating a 120 V will draw a current of 8.33 A. So, for any given conductor size, the voltage drop (Iload x Rconductor) will clearly be greater.
As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.
Before you do any work yourself,
on electrical circuits, equipment or appliances,
always use a test meter to ensure the circuit is, in fact, de-energized.
IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB
SAFELY AND COMPETENTLY
REFER THIS WORK TO QUALIFIED PROFESSIONALS.