|Survival of the smallest, because the smallest is often the fittest.
Bigger things aren't stronger. If size increases; the forces grow, but if the strength of the materials do not increase, bigger equals weaker. "Ants are very strong" everybody knows, but scale an ant up 100 times and it will not be able to lift its own weight. Ants are small; thus strong.
If braking one part means the whole thing is broke, than things made from more parts, complex things, have more chance to fail. (If not able to repair itself and prevent parts from failing by early replacement and maintenance).
Small things use less space, so there can be more of them. The larger the number, the more will survive a disaster. (unless the large number is the cause of the disaster).
A bigger brain is not necessarily equal to more a more intelligent one. Although a bacteria has no room for much brain, big brains can be real dumb or an extraordinary freaking mess.
And not to forget the fucking story: Replication. Small things grow from zero to full size faster than big ones. Faster replication means faster evolution and faster adaption to a changing environment.
Long live the bacteria!
ps. The egg came first; from a sick turkey.