The constant _DATA_SOURCE_MAX_SIZE defines the maximum size of data that can be retrieved by a DATA loop. By default, this constant is set to 2 MB.
It is possible to customize the value of the constant _DATA_SOURCE_MAX_SIZE in your file config/mes_options.php (see the article dedicated to it).
// allow DATA loops to retrieve 3 MB of data
define('_DATA_SOURCE_MAX_SIZE', 3*1048576);