What does rep by pop mean?

It is a principle of democracy that each member elected to, for example, the House of Commons, the US House of Representatives, or a state or provincial legislature, should represent (as nearly as possible) the same number of people.

That is why there is a "redistribution" of federal ridings in Canada every ten years. The boundaries of many federal ridings are redrawn to reflect population shifts and increases.

While rep by pop is a principle, there are many variables in practice. Rural ridings usually have a smaller number of voters than urban ridings. Because of Canada's constitution, Prince Edward Island is entitled to four members in the House of Commons, even though it would not be entitled to that many on a rep by pop basis.

The movement toward rep by pop was promoted in England, back in the 1800s, when several of the ridings had a mere handful of electors but were still entitled to elect a member to the British House of Commons, while electors in heavily populated urban industrial ridings were grossly under represented.

In the US, the members of the House of Representatives are elected on a more-or-less rep by pop basis. But senators are not. The voters of each state are entitled to elect two senators, regardless of the state's population.

There have been calls in Canada that our Senate should be reformed. Its members are now appointed, rather than being elected. They serve until age 75, rather than having to stand for re-election every few years. Some people say the members of the Senate should be elected, but that the number of senators should be a reflection of the number of provinces, rather than being elected on a rep by pop basis.

Is it fair that a state or province with a small population should elect the same number of senators as one with a population that is a hundred times as large? Should someone else's vote be worth a hundred times more than what your or my vote is worth?