I think consideration needs to be given to the size of the net in use aswell. I bought a 3.5m long keepnet because I don't like to see fish crammed in to them for a long period of time, however, at many venues I have to stick to the same 50lb per net limit as other guys fishing with 3m or even 2.5m keepnets.
Maybe a weight limit dependant upon the size of net you are using would be more sensible. It wouldn't be difficult to police as it's easy to see how big a keepnet is, and this sort of rule would encourage anglers to buy the longer, more fish friendly nets.