I can propose two solutions.
First, just as the principle of proportional representation is adjusted for the territories, a slight discrepancy—a reasonable and justified discrepancy that would be statistically marginal—could be established to maintain Quebec's weight at about 25% and end this decline.
However, that requires political courage, because it could in fact spark off a debate about whether that is consistent with the principle of proportional representation. I think that a Parliament with the desire could, as it does for the territories, defend the fact that it's an adjustment and that the principle of proportional representation is essentially respected.
That's a first solution.
However, other means of improving Quebec's political weight in Parliament must be considered.
For example, senators would finally need to be appointed based on recommendations from the provinces. This would finally inject some federalism into the federal Parliament.
The same thing would need to be done for other federal appointments, such as the appointment of Supreme Court and superior court justices. Federalism needs to finally mean something in how this immense power of appointment is structured, a power that the federal government currently holds. In the end, we currently have a Parliament in which the Prime Minister appoints all senators, all judges and all ambassadors, and requires party discipline from most members of the House of Commons. The trend towards the concentration of power needs to be reversed by injecting more federalism into Parliament and, clearly, more democracy.