Section 40(1) of the Supreme Court Act provides that an application for leave to appeal to the Supreme Court of Canada is to be decided on the basis of the importance of the case. 

This is consistent with a core function of the Supreme Court of Canada: settle questions of law of national importance in the interests of promoting uniformity in the application of the law across the country, especially with respect to matters of federal competence.

To obtain leave to appeal to the Supreme Court of Canada from a sentence imposed, varied or affirmed by a provincial or territorial court of appeal, an applicant must demonstrate, to the satisfaction of the court, that the question raised, by reason of its public importance or the importance of any issue of law or of mixed law and fact involved in that question, is one that ought to be decided by that court or that it is, for any other reason, of such a nature or significance as to warrant a decision by that court.

While the Supreme Court of Canada has jurisdiction under s. 40(1) of the Supreme Court Act to assess the fitness of a sentence (that is to say, the quantum of a sentence), as a matter of policy the Court has decided, as a rule of its own making, that it should not do so. It deals with principle, not fitness.

R. v. Boussoulas, 2018 ONCA 326, at para. 15;

R. v. Gardiner (1982), 68 C.C.C (2d) 477 (S.C.C.), at pp. 506-507.
Supreme Court Act

Appeals with leave of Supreme Court

40(1) Subject to subsection (3), an appeal lies to the Supreme Court from any final or other judgment of the Federal Court of Appeal or of the highest court of final resort in a province, or a judge thereof, in which judgment can be had in the particular case sought to be appealed to the Supreme Court, whether or not leave to appeal to the Supreme Court has been refused by any other court, where, with respect to the particular case sought to be appealed, the Supreme Court is of the opinion that any question involved therein is, by reason of its public importance or the importance of any issue of law or any issue of mixed law and fact involved in that question, one that ought to be decided by the Supreme Court or is, for any other reason, of such a nature or significance as to warrant decision by it, and leave to appeal from that judgment is accordingly granted by the Supreme Court.
Stuart O’Connell, O’Connell Law Group, www.leadersinlaw.ca (All rights reserved to author).