You ask how we could compete vs better teams, and then you say it must have been 6:1. Well, how many teams could beat Southampton 6:1 at their ground? Or "just" 4:1.
We weren't brilliant between the 1st and the 2nd-goal, but (1) we were without several players (Ederson, Dias, Bernardo, Mahrez, Foden), and (2) the motivation against teams like Liverpool or Bayern will be through the roof. Southampton are good, but this City team has won so much, it struggles at times vs teams like Palace and Southampton for a reason. We have dropped 17 pts so far, 9 from them against Palace and Southampton.
Generally, we play better against the better teams.