My opinion on this for what its worth :-
Think as customers we have to accept JY are a business and need to make a profit. I would imagine each holiday there is a break even unwritten number (suspect around 18-20 people on a touring holiday) and if the holiday looks like it wont make this mark JY cut their losses and cancel.
We have to remember on top of the coach we are splitting the cost of a tour manager and also paying towards the running of the head office and all other overheads.
Of course they only try to organise holidays they think will pass the break even point as cancelling is a big decision and wont do their reputation any good, suspect it is likely to put some people off hence lose customers.
Now i believe if the group reaches the 25-30 mark that will be considered a reasonable profit level that JY will aim at and in my opinion the ideal number on a coach trip.
Now I've been on trips with up to 45 people and i believe this is a big profit margin for JY. Now i suspect that JY would say these big profit hols counter the ones with a small margin but to me it just makes everything run slower (getting on/off coach, hotel keys, luggage to room, etc) which does impact the holiday.
I may get shot down for this but it would be a nice jesture from JY in the case of these big profit holidays to give the customers a discount voucher for their next holiday (they used to do this on all holidays up to a couple of years ago).
Mick