By DoCmd I presume you are using the DoCmd.TransferSpeadsheet method. If you are importing the Excel file into a new table, Access does indeed check a few rows to determine the data type of each. If on the other hand you create a table with the desired field types, and then import into an existing table, it should import the complete memo field for each row.

I think it would help understand your situation if you could post the code you are using to implement the import and export as well as the SQL string for the query you are using to compare the two tables.