Your transactions (that is, inserts, deletes, updates) are using RBSBIG, but this does not mean that at the same time your package is not using other RB segments.

If other sessions are changing data you are reading in your cursors, then the original data is stored in rolback segments those other sessions are using. For your select cursors to obtain read consistent view of those changed data blocks, they have to obtain them from the corresponding rollback segments. If those other sessions transactions are commited in the meantime, the original images of the changed blocks in the rollback segments can be overwritten by other transaction and your cursor will be unable to obtain read consistent view of the data and will return ORA-1555.