In my opinion, I think Barcelona's coach prepares his team on how to beat these big teams, big teams are attacking minded, so the players focus is always to attack so they can score. He builds this high line defence to neutralise this attack by playing these sharp attackers offside. In that Real Madrid game Vs Barcelona, Real Madrid had 12 offside calls, Mbappe alone recorded 8. So if your team tries to pass the high line, boom offside. These small teams waits for them to attack them exploit the spaces not to be offside.
If they can trash big teams why do you think they can’t beat those smalller teams..besides, if you check the matches they lose, most of it comes out with a result of Barcelona performing better than the team.. why??
You see , the current season Barcelona are performing good and much more better than they used to but what I think might be affecting them is that they feel relaxed whenever they are playing these smaller teams especially the defense and some of the strikers( remember they are performing good so this might not have effect in the stats just that they feel relaxed), they waste more opportunities unlike playing bigger teams which they become more serious ERO secure as many as possible goals to secure the game. This is my view on it actually..