If I had a dollar for every tiresome sermon I ever heard about the damn mustard seed… Like all the parables, it obviously means something else, but without any context like the original readers would have had, one can press any meaning one likes into some of these parables. The mustard seed could represent faith, or hope, or luck, or chance, or will, or determination, or effort, or conformance. But we have just been thinking about topics of seeds and sowing, and about the Kingdom of God, so it’s not a coincidence that we’re talking about yet another seed upon the soil.
The decoder ring says that this parable describes the Roman Empire. It says the ‘seed’, which is Roman culture, was “smaller” than all the other seeds upon the soil. I’m not certain how size relates to culture in the context of seeds, but I can guess that this was some reference to the relative youth of Roman culture compared to many in the East, which would produce much larger “seeds”. This seed becomes a tree with great branches under which birds can nest in the shade. I think it would follow that the message is that the Roman Empire is like a great tree that protects all the nations it embraces.
The bit about birds in the air nesting in the shade comes from a reference to Ezekiel that’s repeated in Daniel. To wit: God plants a great tree that all the birds of the air can nest within. Again, we see a message with several levels of interpretation possible, depending on perspective (and literary history). And then we see the narrator explain that Christ was explaining all this to the disciples while deliberately obfuscating his message to everyone else.
This is explained that Christ was “speaking the word” to them as far as they were able to hear it. He was teaching them as much Roman culture as they were able to understand it. The Judeans were clearly defective since they didn’t easily accept Roman culture. He could only explain so much, since they were defective.