"If the site is full then how can it accommodate more people?"
The campground ostensibly has a number of slots, which are of a certain size usually to accommodate a car + caravan. You could easily fit a number of cyclists on even just one of these lots. Also, there are always areas which are kind of "nowhere in particular" in these park campgrounds, between the trees on the edge or behind the office etc. Being "full" isn't such a black and white concept in reality, everybody who has been to one of these places knows that there is always a spot somewhere to put a small tent without bothering anybody. All this petition is trying to do is make the idea more official, rather than being at the whim and mercy of whoever is in charge that day. People on bicycles can't simply drive another 10 miles to the next campground when it's 8pm, and you're not allowed to wild camp in the park proper, so there aren't a lot of options here. It just makes sense to say that cyclists should always be able to find a spot, in my opinion, and it wouldn't detract from anything else in the camp to do that.
"How is the capacity of a site determined?"
You put aside an area for the hikers and bikers, and say to them "Find a spot". It's not that difficult, really. If it gets more packed then people have to move a bit closer together, maybe, but again, it's not like you ever get unexpected hordes of hikers and cyclists descending on these places - maybe a group occasionally, but there could always be a condition that says commercial tour groups have to book ahead. This is meant for your regular cyclists who aren't supported.
"Should sites be required to keep 3 or 4 pitches free for late bike campers in the same way that carparks are required to keep a few bays for disabled users?."
You don't have to think of it in terms of formal pitches. Just put an area (that is probably unusable for car camping anyway) aside and let people sort it out amongst themselves. When I camped in these parks, it was often a place under some trees behind the office or whatever. You just wing it, it's quite informal in terms of layout - but the idea here is to formalize the concept of this area existing at all. After that, leave it up to people how they arrange the space between themselves.
"Using the disabled parking analogy, although those bays are reserved specifically for disabled users, once they have filled up the car park dosent have to provide additional spaces because its full."
No need to take up any disabled spaces, or spaces that could be used by car campers. Most of the time, hiker-biker sites are not usable by car campers because they are away from the open areas, amongst trees, and no hookups, fire pit etc.
"Its just that once something is full, its full."
If you chop a campground up into sites sized for car camping, yes, that's true. But not true for cyclists, who mostly just need a place to put their tent on the ground and go to sleep. The concept of "full" is just not that black and white in real space terms.