As I unterstand this was a limitation of V2 (http://effbot.org/zone/sqlite-blob.htm):
Does this limitation still exists in ZEOS?The SQLITE database has limited support for large binary objects (BLOBS). There’s a limitation of 1 megabyte for each row of data, and the database uses NUL bytes to separate columns in the storage.
Note: This limitation has been removed in SQLITE 3.0.