This is most likely down to your PHP configuration.
Namely POST_MAX_SIZE and UPLOAD_MAX_FILESIZE. Maybe even the time limit allowed for the script to run.
If your host allows you can change this with some PHP at the top of the page in question.
<?php
ini_set('post_max_size', '64M');
ini_set('upload_max_filesize', '64M');
set_time_limit ("360");
?>
Hope that helps?
Cheers
Ian