You should also check their performance in their own domestic league in this season, both Bologna and Girona performance are terrible, while Leverkusen is still fine even though not as strong as in the last season. But, Leverkusen met easier opponents, like Milan and Feyenoord, which is nothing surprising they can won those match.
After all we still have many upcoming match, Girona is quite lucky their opponent not that strong.
Bayer Leverkusen is quite better than Girona and feyenoord put together in terms of performance and results this season, Girona are first timers in the champions league and are not used to playing in this kind of competition whereas Feyenoord have not been frequent in the competition as a result of their shortcomings due to their failure to qualify for the tournament, though Bayer Leverkusen also had similar issues but not until they contracted the Spanish coach who came in turned things around and now they have reclaimed back their status as one of Europe's elite club and haven't really disappointed the fans yet on their quest to conquer Europe just as they did in the German bundesliga and I won't be surprised if that happens because in football a lot of miracles do happen.
I would say Leverkusen shouldn't be compared to teams like Feyenoord and Girona. Feyenoord team is relatively weak. Girona's squad, on the other hand, was strong last season. But Girona's squad is not strong this season. And this is why we are seeing erratic and poor performances from Girona. Girona's performance in La Liga is also not strong. At the same time, Girona lost both of their Champions League matches.
Leverkusen have continued their excellent performance in Champions League matches so far. In the first match of the Champions League, the Leverkusen team managed to win against Feyenoord with a big margin of 4-0. In the second match, Leverkusen faced an experienced team like AC Milan. Leverkusen won this match 1-0. Leverkusen is expected to win easily against Brest in the next match.