In https://github.com/paritytech/ink/blob/762730c63daa11f1f3349f5f6d43c77ca7468ddb/crates/storage/src/lazy/mapping.rs#L174
try_insert() check key.length() + value.length() <= 2^14
Then calls insert() , but in insert() inserts the tuple (&KeyType::KEY, key) where KeyType::KEY is a u32, so needing to serialize set_contract_storage which is something of length key.length() + 4.
In https://github.com/paritytech/ink/blob/762730c63daa11f1f3349f5f6d43c77ca7468ddb/crates/storage/src/lazy/mapping.rs#L174
try_insert() check key.length() + value.length() <= 2^14
Then calls insert() , but in insert() inserts the tuple (&KeyType::KEY, key) where KeyType::KEY is a u32, so needing to serialize set_contract_storage which is something of length key.length() + 4.