Chelsea were being offered very high odds which were more than 5x but PSG were offering very low odds because PSG had more supporters. On one hand PSG is a good team on the other hand they were offering low odds, seeing which most of the gamblers got confused and bet on PSG with confidence which led to their loss.
I don't think it's because of the number of supporters, no (and it wouldn't be very lucrative for the booker to go by this figure either).
PSG had a good preliminary round and excellent quarter-final and semi-final matches, whereas Chelsea were almost eliminated beforehand and also had a much easier draw. PSG are also the current CL winners and dismantled Inter in the final, so they were rightly the (heavy) favorites here too.
I wouldn't say that odds are created based on the number of supporters. Real Madrid certainly has 50x more supporters than PSG, so the odds were still on the side of the Parisians.
It is not uncommon for a team, after easily defeating the 'toughest' opponents, to relax a little because the next game is against an 'outsider'.
Chelsea is one of the most difficult teams to predict. They can beat anyone, even the best teams. They can also lose embarrassingly to anyone.