You are only getting the first 256 [or whatever display setting in the table structure] of the memo field. To get more you may have to run a query which creates two or more Alphanumeric fields each containing the 240 character's from the memo field. The calculation would be Left(memofield,240) , substr(memofield,241, 480), substr(memofield,481, 720) and so on. prior to exporting.
There may be another way to do it but I haven't found it yet. Especially, since there is no limit to the size of a memo field.