If you have a lot of crappie under 5", and not many larger specimens, especially considering your bluegill are mainly 2-4", the pond is not in balance. Right now it's ideal for big bass, and not ideal for anything else. The catfish will eat some crappie, but I would think they're eating more small bass right now since bass spawn later than crappie and the crappie grow past the size the catfish can eat them earlier in the year than the bass do.
I would think that you'll get bigger crappie if you thin out the catfish and let the bass overpopulate. You won't have as many large bass, but the larger numbers of bass will hammer the YOY crappie as soon as they're hatched. Keeping every crappie you catch will also help.
Another option is to stock a handful of tiger muskie. They get big enough that they would thin out the small crappie and bluegill both, allowing both species to grow significantly larger; they would also eat a good number of small bass, so your bass would average larger than if you rely on overpopulated bass to thin out the crappie and bluegill. Muskie are eating machines, basically, and would serve to keep other species from overpopulating.
