Oracle data transfer rate determined by rows instead of data size
More likely limited by the fetch size, which is a client side setting.

For example: https://xzwang.wordpress.com/2013/06/20/oracle-ole-db-performance/

Quoting some content of the blog post:


  Batch size controls how many rows SSIS retrieves with each “batch”. 
  After retrieving all the rows in a batch, SSIS signals to Oracle that
  it is ready for another batch.   Of course, network latency is
  introduced during this signaling process.  Thus, decreasing the number
  of total batches also decreases the number of round-trips SSIS has to
  make.  The default batch size is only 100;  increasing this to a
  reasonable number should boost performance.


And:

https://scottadamsbi.wordpress.com/2010/11/02/sql-server-analysis-services-and-oracle-ole-db/


  If you are experiencing the same or similar issue, then you may wish
  to experiment with different settings for the FetchSize parameter on
  the Oracle OLE DB connection string e.g. Data Source=ORACLE.WORLD;User
  ID=User;FetchSize=100000;. The default setting is FetchSize=100. This
  means that the Oracle OLE DB provider will request and fetch 100 rows
  of data. SQL Server will then read 100 rows of data. Once all 100 rows
  have been read, the Oracle provider will request another 100 rows of
  data. This will continue untill all rows are read.