It has been based on studies...and the reason it changes is as studies develop over time
You can read about some of the latest research in this RSPCA document (page 2):
http://www.rspca.org.uk/ImageLocator/LocateAsset?asset=document&assetId=1232725725160&mode=prd
I'm glad that rescues etc. are moving with the latest recommendations and I'm glad that, in general, we mostly seek to make our enclosures as large and as stimulating as possible. If we didn't, we'd all still be keeping rabbits in 3ft hutches and rescues would still be accepting that as suitable, and we all know it isn't. So of course they have to keep up to date with latest welfare research when determining whether a home is suitable.
I've used the analogy before - if I really, really wanted a horse, could afford a horse, knew what to feed a horse, had a great horse vet, and would absolutely love it to bits, but I didn't have the recommended amount of space and could only keep it in my dining room...should I get a horse? No, I shouldn't. ALL the welfare requirements are necessary in order to meet the animal's needs...it's no good you having food but no water, or a clean house but no food, for instance. It's ALL relevant and it's ALL necessary.
I do agree it's really tricky when guidelines change, as there will inevitably be a transition period when we review our accommodation to see if it still meets needs (also bearing in mind individual circumstances such as the age/disability of any bunnies), but overall if new research and evidence suggests we've been doing things wrong, then I do think we should make efforts to adapt for the good of our bunnies. Just because bunnies seem happy how they are now, doesn't mean that they might not be happier with changes. It's always going to be a pragmatic decision though isn't it - if there is absolutely no more space at all for instance, and nowhere else to adapt things to, then assuming the space is still relatively large (i.e. met the previous guidelines), it's probably better to keep things as they are than to throw hands up in the air and rehome the bunnies, for instance.
I think products like runaround have really helped in recent years, as they provide greater flexibility for people in providing accommodation in ways and places where things might not have fitted previously. It makes upgrade-itis a little easier :lol: