I think that the level of the competition can also set free a lot of mental elements that otherwise may not contribute as much to the game of a team. Real Sociedad may not be one of the strongest teams in La Liga at the moment, but they are certainly not one of the weakest teams either. If they have most of their important players available for the games against PSG, I think that the game could go either way (or the two games in aggregate for that matter). PSG should have some advantage over them, but even if you say that PSG is a 60% favorite, there is still a lot of room left for Real Sociedad to be successful against them in the Champions League.
To be honest, I don't see any chance for Real Sociedad to beat PSG. They simply lack the strength to take on the superstars of PSG.
PSG has avoided most of Europe's elite clubs, including Bayern Munich, Manchester City, and Real Madrid. All are heavy opponents of the French champions, and if they meet these opponents, they can end their Champions League race early. They are lucky to only face Real Sociedad. Facing Real Sociedad is said to be "easy" in the 1/8 round of the Champions League, but Paris Saint-Germain still appears cautious and highly appreciates the opponent. No opponent is weak once entering Knock-out. Moreover, it is important to remember that Real Sociedad competes at the top of La Liga. This is an opponent that PSG needs to prepare carefully for. This year's PSG squad is not outstanding, as they only try to finish second in Group F.