Ask HN: How do you reject a startup job offer without burning bridges?

In the past month or so, I've had job offers from two wonderful startups (co-incidentally both are YC companies). While I've found both the companies and the teams highly motivated, I made my decision to join the slightly larger of the two startups, mostly because I come from a large enterprise software company and probably don't have the confidence to join the smaller one at this stage.

Now, despite of me expressing the reason for rejection to the smaller company, they have been trying to convince me that I might be backing out due to imposter syndrome, which is clearly not the case. It has been three days and despite of me rejecting the offer, the co-founders of the smaller company have been calling me in.

Given the situation, how do I reject the offer without burning bridges ?