We are sorry that you were not completely satisfied with the tour, I would like to inform you that the tour was not organised by viator, viator is only a website where you can book tours, a bit like booking.com for hotels, the 2.99 you paid for the booking through viator is their commission for allowing us to be on their website, then original florence which is the cultural association that organises the tour uses part of the donations from clients (2.50 per person) for the promotion and publicity of the events.
Regarding the size of the group, being a tour based on free donations, it is normal that we need large groups to allow the guides to make a profit. If you do not like tours with groups of 20 people, I recommend that next time you take a private guide, but I must inform you that for a 2 hour and 15 minute tour ,they get 120 to 150 euros, or alternatively you can book a tour at a normal price with a tour operator, where the prices range from 25 to 50 euros per person and most likely you would be in a group of 20 people anyway.