Loading Japanese Characters

All,

I am attempting populate a DB2 table with data (containing Japanese characters) from an Oracle table using Informatica.  The Oracle and DB2 system are properly configured to use UTF-8 (as is Informatica); I am capable of  viewing the Japanese characters in each of the Databases as well as in Informatica. Now, when I attempting to populate a field in a DB2 table with the contents of a field in the Oracle table with the exact same size:

Oracle varchar2(128) to DB2 varchar(128)

I am recieving the following error:

[IBM][CLI Driver] SQL0302N  The value of a host variable in the EXECUTE or OPEN statement is too large for its corresponding use.  SQLSTATE=22001
 sqlstate = 22001

What I did notice about the data causing these errors was that the length of the (Japanese) character string was always greater than 64 characters. It is my understanding that Japanese characters require double the amount of space versus standard 7bit ASCII. As a temporary solution I have had my DBA double the size of the field, however I would like to know a reason as to why this happening. I would be curious to see what some of you guys have to say about this issue. Thanks.