There is no limit for any club to achieve big wins, as long as they are able to perform optimally and always make things difficult for their opponents during the match, any club has the right to achieve big wins because this is a bonus for their hard work so far. Arsenal is heading to the next level, Arteta has succeeded in making Arsenal look better than before. They deserve what they want as long as they are able to maintain their performance, Arteta must work even harder to bring Arsenal to a prestigious trophy.
Last season they almost won the EPL trophy from Manchester City, but due to not being able to maintain their performance towards the end of the season, Arsenal had to lose the long awaited title. Arsenal is one of the teams from the EPL that almost every season always qualifies for the Champions League, this season they will try to do something surprising, new players are starting to be brought in to increase their strength to compete with other clubs.
Any team could end up winning, it is not about how good you are ,but to be fair big teams winning is of course the most often common situation. I think it's clear that we are talking about something that will not be all that simple, we are talking about something that will be quite strongly competitive. Arsenal could win, it's not impossible, they are not a terrible team, but when you have City, Madrid, Bayern and such there, it's going to be very tough.
Chelsea won it over those teams a few years ago, when they weren't even doing that great at premier league, which shows you that it's possible to do fine even if you are not the biggest club, it's just that after them real won twice and city won once, which shows you that the odds of a smaller team to win is difficult. And by "smaller" I mean Arsenal level, a team that hasn't won it in god knows how many years, maybe never, don't know if they ever win an UCL before.