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.
Real Sociedad is one of the big teams in La Liga. They often finish the season in the top 6 of La Liga standings. I agree if we can't underestimate Real Sociedad because they have the capability to make a surprise. We know if the teams from La Liga isn't an easy team to be defeated. They often win international competitions like UCL and UEL. So, PSG must be careful to play against Real Sociedad in the upcoming match. Moreover, if Real Sociedad can play with full team. They have the potential to defeat PSG and qualify for the next round. However, I still trust PSG to win the match because they have more quality players.
I don't think real socieded is too big a team. league winners are rare, getting top 4 is also very rare. only last season got top 4 so this season can compete in the champion league. it's been a long time since real socieded competed in the champion league.
In my opinion, the 3 biggest teams in laliga , Real Madrid, Barcelona, Atletico Madrid plus Sevilla. talking about trophies, they also get a lot of titles both in local competitions and European competitions.