Signs of the birth of a big project

Analysts have situations where the customer brings a small task and hopes that he will quickly get the result. The analyst studies the task, asks questions and the task grows into a big project. Or into a significant and promising project, but "sounds like years of development". And this all has begun with a request for a small modification, which should solve the minor pain of the customer. I was curious how does this happen? In my talk, I want to speak about from where and why such big projects appear, and what to do with them.