Reapportionment

Apportionment is the process by which seats in a legislative body are distributed among administrative divisions entitled to representation.

The simplest and most universal principle is that elections should give each voter's intentions equal weight. This is both intuitive and stated in historical documents such as the Fourteenth Amendment to the United States Constitution (the Equal Protection Clause). However, there are a variety of historical and technical reasons why this principle is not followed absolutely or as a first priority.

Fundamentally, the representation of a population in the thousands or millions by a governing body that is much smaller involves arithmetic that will not be exact. Although it could make representation more exact for a representative's vote in the governing body to be weighted according to the number of his constituents, it avoids complexity and awkwardness if each representative has exactly 1 vote.

Over time, populations migrate and change in number, and preferences change. Governing bodies, however, usually exist for a defined term of office. While Parliamentary systems provide for dissolution of the body in reaction to political events, no system tries to make real-time adjustments to reflect demographic changes. Instead, any redistricting takes effect at the next scheduled election.

In some representative assemblies, each member represents a geographic district. Equal representation requires that districts comprise the same number of residents or voters. But this is not universal, for reasons including the following:

A perfectly apportioned governing body does not ensure good representation; voters who did not vote for their district's winner might have no representative who is disposed to voice their opinion in the governing body. Conversely, a representative in the governing body may voice the opinions held by a voter who is not actually their constituent, though representatives usually seek to serve their own constituents first and will only voice the interests of an outside group of voters if it pertains to their district as well or is of national importance. In any case, the representative does not technically represent the voter as the elected official from their district.

...
Wikipedia