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As others have alluded to, seats in the House of Commons are either guaranteed by senatorial clauses (involves changing the Constitution) or various Amalgam Formulae.

1915 – The senatorial clause
In 1915, the first change was made to the original representation formula, by the adoption of the "senatorial clause." Still in effect today, this clause states that a province cannot have fewer seats in the House of Commons than it does in the Senate. In 1915, it had the immediate effect of guaranteeing four seats to the province of Prince Edward Island, instead of the three it would otherwise have had. It has had four seats ever since.

1974 – The "amalgam" formula
In 1974, concern over the continuing loss of seats by some provinces prompted Parliament to adopt the Representation Act, 1974, which, among other things, guaranteed that no province could lose seats.
In February 1974, the Standing Committee on Privileges and Elections established that:

The objective must be adequate and realistic representation of all Canadians bearing in mind the historic undertakings arising out of Confederation and its responsibilities. The allocation of seats (in the House of Commons) is at the very heart of the Confederation compromise.

A "compromise" was therefore proposed to deal with representation in the House of Commons. The new formula, the third in our history, was more complicated than earlier ones. As in the pre-1946 rules, Quebec was used as the basis for calculations, but there were three differences. First, Quebec would henceforth be entitled to 75 seats instead of 65. Second, the number of seats assigned to Quebec was to grow by four at each subsequent readjustment in such a manner as to slow down the growth in the average population of an electoral district. Third, three categories of provinces were created: large provinces, those having a population of more than 2.5 million, intermediate provinces, namely, those with populations between 1.5 million and 2.5 million, and small provinces, with populations under 1.5 million. Only the large provinces were to be allocated seats in strict proportion to Quebec; separate and more favorable rules were to apply to the small and intermediate provinces.

The amalgam formula has been applied once, leading to the establishment of 282 seats in 1976.

THE PRESENT FORMULA
Following the 1981 census, calculations revealed that the amalgam formula would result in a substantial increase in the number of seats in the House of Commons both immediately and after subsequent censuses (369 seats were projected after 2001). Effectively putting a hold on the process already underway to reassign seats, Parliament passed the Representation Act, 1985. It came into effect in March 1986.
The adoption of the Representation Act, 1985 greatly simplified the formula described in the amended section 51 of the Constitution Act, 1867 for calculating representation.

The current formula for representation is applied by carrying out the following four steps:

1 – Allocation to the territories
Starting with the 282 seats that the House of Commons of Canada had in 1985, one seat is allocated to the Northwest Territories, one to the Yukon Territory and one to Nunavut, leaving 279 seats. This number is used to calculate the electoral quotient.

2 – Calculation of the electoral district average
The total population of the ten provinces is divided by 279 to obtain the "electoral quota" or "quotient", which is used to determine the number of seats for each province.

3 – Distribution of seats to each province
The theoretical number of seats to be allocated to each province in the House of Commons is calculated by dividing the total population of each province by the quotient obtained in step 2. If the result leaves a remainder higher than 0.50, the number of seats is rounded up to the next whole number.

4 – Adjustments
After the theoretical number of seats per province is obtained, adjustments are made in a process referred to as applying the "senatorial clause" and "grandfather clause."

As we have seen, since 1915, the senatorial clause has guaranteed that no province has fewer members in the House of Commons than it has in the Senate. The Representation Act, 1985 brought into effect a new grandfather clause that guaranteed each province no fewer seats than it had in 1976 or during the 33rd Parliament.