Are irreversibility and unpredictability in a system caused by the huge number of molecules?

In my understanding, if we imagine two molecules in a system, we can predict what’s going to happen and it’s easier to reverse the process than to predict the motions of billions of molecules and reverse them.

Given that this is correct, is there anything else involved that make the prediction and reversion of microstate changes in a thermodynamical process or is our biggest hurdle only the calculation part?