The concept of bigger/smaller is useful but is a distinct skill from counting. If you spread the M&Ms apart enough that the part of the brain responsible for gestalt clustering can't group them into a "bigger whole" signal, they'll no longer be able to do the thing you're saying (this is the law of proximity in gestalt psychology).

Most animals can distinguish bigger from smaller.

However many animals can distinguish independently small numbers, like 3 or 5, and recognize them whenever they see them.

So in this respect, there is little difference between humans and many animals. Humans learn to count to arbitrarily big numbers, but they can still easily recognize only small numbers.

> many animals can distinguish independently small numbers, like 3 or 5

This is called subitizing. It's distinct from counting. We can see the difference in humans with Simultanagnosia, who are unable to count beyond the subitizing range. Subitizing is categorizing the scale of a small gestalt group.

The only thing I've ever seen where an animal appeared to demonstrate counting (up to 3) without training was in rhesus monkeys (maybe also chimpanzees?), but even that experiment could be explained through temporal gestalt. (It's the only reason I know of for them to not have been able to go higher than 3 in that experiment in the context of many other things that they can do.)

Even parrots can count to 6 and more, I would be surprised if primates couldn't.

At least one has maybe been shown to be able to do that with 30 years of focused training, but none have been shown to be able without training. Wild parrots have only demonstrated subitizing and size discrimination, not counting.

The overeager do quite often confuse subitizing and size discrimination for counting, though. That's its own problem.