In most pubs a large menu is simply a sign that most of the food comes shipped in pre-pared and frozen, and then re-heated to order.
That's not always true - if a pub has a good throughput of food, then it can be all fresh. But if you see a pub with a small-ish menu, that's generally because they're actually cooking stuff rather than re-heating.
It's quite sad that eating out often involves all the skill being in some sort of central factory.