All members of parliament represent the same number of citizens
In Regional Systems
In Single Member regional system where members are elected to represent their own region the issue is straight forward. If the population size is equal in each region the representation is said to be balanced. The representative in for each region is expected to represent all the members of the region independent of if they had voted for them. That does not mean that they must do what they say but that they must grant petitioners an audience. However, this is rarely enforced so some claim that citizens who disagree with their representative are not truly represented.
This situation is more complicated in a Multi-member system. In such systems each person is represented by multiple representatives. This is both a benefit and a detriment to the issue of representation. On the positive it is more likely that each citizen can find one or their representatives which agrees with them on the topic they want to speak to them. On the negative each member is less accountable and can simple pass the petitioner to the another representative. This concept of Petitioner Accountability is a common topic of debate among election scientists.
In Partisan systems
In a partisan system several members of parliament are elected to represent parties and none to a region. In a Party List system no citizen can really be said to be represented by a member of parliament unless they are a member of a party. Very few citizens are member of parties so this means that there is no grounds on which to even define Balanced Representation.
Citizens are represented by parties not members of parliament. The term for if this is balanced is Proportional Representation