Most are saying that if you hold big amount BTC you choose paper wallet. what is the reason behind that?
Because paper wallets are considered cold storage whereas compared to a wallet on your personal phone or computer, your funds should be totally safe from hackers assuming you created the paper wallet correctly.
In general, I'd recommend hardware wallets than paper wallets.
and just want to know if i keep small amount BTC on paper wallet , what is the disadvantage?
1. If you planned on making a transaction, you need to create a new paper wallet after the fact for security purposes.
2. Your funds can be stolen if you created the paper wallet using a malware-infected device.
3. Paper is fragile (though you can easily create multiple copies).
Hard ware wallets are too expensive and the setup process can be quite tedious for newbies.