Age is the key thing in this. I hate to say it but based off my lifetime and recent history but Birmingham are a bigger club than Forest. They've been in the top flight more and have won a cup. I don't think of them as a big club but shows how subjective this is.
I also don't like the idea of recognising clubs as ones that deserve to be top flight. If any club makes it to the top flight they deserve to be there and all fans dream of their club being top flight.
It's only a small step away from saying that only certain clubs should be in the European competitions and that's something I think we'd all challenge.