Most are saying that if you hold big amount BTC you choose paper wallet. what is the reason behind that?
These days there aren't many people say that since hardware wallet and offline/air-gapped computer are more popular. As for the reason, it might be because it's cheap and secure option
only if it's done correctly.
and just want to know if i keep small amount BTC on paper wallet , what is the disadvantage?
Some inconvenience since,
1. You need to import the private key to another wallet in order to spend the coin.
2. After you import the private key, it's not recommended to use that paper wallet again due to security concern.