It's not as much a case of "big" vs "small", and in fact IMHO that's the over-simplification you're doing.
All those examples are a question of configurations and interactions between the pieces. That's what creates something else.
To take them in order:
A) glucose. It's not just big candy from small atoms. It's that a very specific ring configuration can bind to the relevant proteins on your tongue, to trigger the "sweetness" signal. The atoms on their own, or most other configurations of the same atoms, don't react with that protein at all.
B) rust. Metallic iron is just a reflective grey, oxygen dioxide is transparent, but a certain iron oxide mollecule has the resonance frequencies just right to look red.
C) thermite. Because in that combination the iron oxide is actually the oxidizer, wheras if you tried to burn iron oxide in the air it would be the fuel and since it's already oxidized, it doesn't burn any further. It's the transfer of oxigen from the iron oxide to the aluminium that creates that huge amount of energy when thermite burns.
D) the computer. Well, here we're getting into hardware design and I really don't know how to explain it simply. But let's try.
Let's think of a simple structure which can hold a 1 or a 0. One such is a bistable circuit. (Has 2 stable states, hence the name.) A.k.a., a flip flop. You need at least two transistors to make one.
The secret there is that when one transistor is conducting, it applies a voltage on the other transistor's gate that opens the circuit there. And viceversa. In more than one way. Because the other transistor isn't letting current through, the first one's gate doesn't get electrons on it, and it can continue to let current through.
Awful explanation, I know. But ayway: It's the combination of two transistors which can store a 1 or a 0, while each of them on its own can't.
(Well, there are many other ways to store a 1 or a 0, some with just a transistor, but then they need more transistors in some other circuitry one one way or another too. Anyway, the above is typically what you have in your CPU.)