Slightly beyond ELI5, but I have some follow-up questions.

But they skip the parts about needing to perform certain functions it doesn't need to do, and uses that space to pile in even more circuits for what it does need to do.

The most common example I've seen here is that GPUs forego circuitry related to branch prediction and out of order execution in order to fit in more FPUs.

What else is foregone? An AMD CPU has 8 cores with 32 FPUs, 32 APUs, a couple of renaming units, and then "other stuff" like cache, branch prediction, etc. But a GPU has over 1,000 FPUs. The physical dies are similar in size, right? Do cache and branch prediction really take up that much physical space on the die?