What's the difference between 'big' founders and 'small' founders that makes you think the former should be pursuing different ideas? If a 'big' founder thinks

What's the difference between 'big' founders and 'small' founders that makes you think the former should be pursuing different ideas? If a 'big' founder thinks an idea will be successful, and there's a market for it, and he/she wants to bootstrap it, and it's something they'd be interested in, why shouldn't he/she pursue it? Why do they need to go for something more ambitious like a climate change tech startup?