When starting from a smaller account it takes some time in the beginning to build up account trust and social proof. If you start from a larger,more established account, you have a head start. People are more likely to trust you and follow you at a higher rate.
If your account has less than 1000 followers users tend to not follow back as much as they would if you have a larger account. Think about it, would you follow an account with barely any followers? Also, newer accounts must be treated with care as accounts with lower than 1000 followers have a low trust score early on.
So, if your account has less than 1000 followers we have to follow your target audience much slower than norma + keep in mind that when we do bring back users to your accounts a % of them are unlikely to follow you because you do not have a lot of followers (social proof).