I can see two sides to this. We run shows and have a few ponees some of which are with professional producers.
I definitely think that there have been lots of issues with over weight riders and novice riders riding ponies which are too small for them. it looks awful and certainly in a class I'd be marking them down as the "picture" would be all wrong and the ponies would normally not go well.
I think it's particularly bad in M&M classes where the adults have not moved onto horses and still try to ride ponies they have long since outgrown.
I also know that lots of professional producers either ride in the smaller ponies to be ridden by their client's young children or get their in house stable jockeys to do this.
Most of these riders are extremely experienced and although tall are often very light and ride so much better than the average riders.
The biggest issue is that most show grounds won't allow lunging now and therefore it is very hard to work in lead rein ponies without a teenager/light adult getting on them first.
People will say that the little children should be able to ride them in themselves. I would say that putting tiny children (3 to 7yos) on Lead Rein show ponies at County shows without any form of working in is a recipe for disaster.
The sort of ponies that producers ride in aren't your average older plods but often 4 and 5yo babies who need some exercise before they go in the ring.
Whilst I think there is definitely a need for this to be patrolled I also think that if adults are stopped from riding the lead reins I think showing will become a class for the steady eddies rather than the ponies with charisma which is what showing has always been about.