Small things do change the course of life and history.
Random encounters. Situational factors that made you happier of angrier and driving a social connection to excellence or to extinction.
The state of mood one got (the garbage machine woke you up at 5:30 am) exactly in the day when a great idea came by, and it went to the garbage can for moody reasons.
The phone call that interrupted writing and cut your strain of thoughts. It can lead to even other and better ideas, or to despair.
Dynamics are even more vicious. Missing the train makes you exhausted, you come with less energy, get an idea you would not get in calm times and so it goes. It may change your whole life.
Small vs. big effects. The non small things have their strong effects, too. They are just less determinal and less general than one thinks.