This is a question about the physics of muffins. With a standard size muffin tray, when filled with batter, the heat and expansion causes the dough to spill over the edge and form the classic muffin top. But with smaller and smaller tray sizes the amount of expansion will eventually just be a bump that does not spill over the sides. What is the theoretical minimum muffin size, assuming standard muffin batter (no upping the baking powder). would it be possible to make breakfast-cereal sized muffins?
First, you must assume a spherical muffin. This is a silly but fun question, so lets think about the limiting factors.
What if you could scale down everything (the pan, the size of the ingredients, the size of the gas cells, everything else) uniformly.
If that could be done, muffin tops would be a uniform phenomenon at any size.
So, what can we not scale down uniformly?
It seems to me that there are to major factors that cannot be uniformly scaled down in actual practice:
Practically, it is likely that the current mini-muffin pan size (a couple of centimeters across) is about as small as you can go without having to start taking drastic measures with the recipe. The main problem is heat management: getting good oven spring without the muffin setting too fast, while cooking it through, and yet having nice crust development on the outside.
You can have smaller baked goods, but they have a much simpler structure. They are called cookies. Most muffin batters would make an effective drop cookie, again probably down to about 2 cm. They would be essentially all muffin top.
For a look at some of these practical factors, see: