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.