It is often argued that one of the desirable features of a democratic election is that every vote should be of equal value. One of the ways in which that principle can be realised is by ensuring that the ratio of representatives to voters is more or less the same everywhere.
In the case of Britain’s single member plurality system, that means ensuring that the number of voters in each constituency is more or less the same. If that is not the case, the votes of those in constituencies that contain fewer registered voters may be thought to have more weight and influence than the votes of those in seats with more voters. And if one party does especially well in constituencies with fewer voters, it is likely to secure what might be thought to be an unfair advantage in terms of its overall tally of seats.