There is a definite energy advantage to being very small. When I was working in the proper nanobot space it became quickly apparent that "gray goo" was nonsense

There is a definite energy advantage to being very small. When I was working in the proper nanobot space it became quickly apparent that "gray goo" was nonsense. Bacteria are grandmasters at throwing away things they don't need. In fact, due to epigenetics, many bacteria don't even contain all the genes they need to survive in their own environment — the cost of maintaining the complete set of genes is distributed through the local population. The corollary is that anything that's small is limited to doing things "like a bacteria would". The only way around the energy barrier is to pump energy in from an outside source; y'know, like plants. Notice that plants don't get around very much because their energy source is pretty scant. Just "dumping more energy in" doesn't really work because the energy scales cause (what we called) 'spontaneous disassociativity of functioning parts'. Personally, I think the real breakthrough was LBL-silicon turing complete silicon nanoparticles. They combine the best parts of nature (proteins) with the best thing we've though of (computers) to provide a pre-defined library of additional capabilities to the host organism.