anomaly bandwidth on data usage over the database link

My app periodically downloads  all the new transactions from a remote database over a database link. The remote database is initialized with new data only at midnight and gradually add more data during the day. Downloaded records are marked on the remote database so that they won't be downloaded the next time.

The difference between the sql of the first transaction ( of the day )  and the one of the other transactions is the first sql does not have the where condition that checks if a record has been downloaded. ( it downloads all transactions the first time )

The first transaction of the day is always the biggest one with about 3000 records, then all the next ones have around 10 - 20 records each time.