Some casinos release small withdrawals instantly and check large withdrawals manually. Large and small are not the only criteria, mate. The casinos have some other security points that could flag the withdrawal for manual check, even if it is small. For example, if they detect multiple accounts somehow, they would flag the withdrawal and ask for KYC before they suspend the account.
I just mentioned one possible reason/criterion. Other possible reasons could flag a withdrawal for manual verification. As for spinly, they approve withdrawal manually regardless of the withdrawal size. Of course, they have some limitations, which is why the withdrawal is manual. But they will change it in the future.
There are possible other reasons like your recent login activities, your bets recently, bet style and your latest win streak, big wins, big withdrawals for example can be combined together and put your account under suspicious investigation by a casino.
As a business, they must put those accounts under investigations as it is wrong to assume that nothing is suspicious or wrong with those accounts. If they don't investigate, their company can be exploited without proper necessary investigation by the company. It's clearly that if investigation result is good for that account, the casino will happy to proceed withdrawal for that user.
Investigations are necessarily against cheaters, not against good users.