Do they? In my opinion, Bayern's squad is definitely not weaker + they play at home. I wouldn't risk a cash bet on Chelsea, although the odds probably make up for the risk, here's what the bookies think of the game at the moment:
1.73 - 4.16 - 4.26
As you can see Bayern are considered big favourites and Chelsea's win would be considered a big surprise. I don't think Chelsea will spring surprises in international games time after time (the last game was against PSG).
The odds are not balanced between Bayern Munich and Chelsea, so I wouldn't dare bet on Chelsea, especially since this is an away game for the Blues.
At the very least, Bayern Munich will be much better than Chelsea. Even though Chelsea won the Club World Cup, that doesn't mean they will win every game.
Just look at PSG, who won the UCL but didn't win the Club World Cup.
And if you remember how PSG started their journey in the Champions League, it is clear that separate games from two months ago mean nothing. Chelsea have had a good start to the Premier League but they have had easy opponents. Bayern look much more dominant. I also think that this factor will play a role: Chelsea is unlikely to build its playoff strategy on the basis of a victory over Bayern, they are concentrating on the EPL. Bayern, on the contrary, will try to get into the top 8, concentrating on the Champions League, since they have everything under control in the Bundesliga.