I got the same problem after upgrading WP from 3.6.1 to 3.8.1.
It looks like there was a change in wp-includes/media.php function wp_constrain_dimensions(): width and height is set to max(1, $current_<w/h> * $ratio). I have changed:
$w = max ( 1, intval( $current_width * $ratio ) );
$h = max ( 1, intval( $current_height * $ratio ) );
to
$w = intval( $current_width * $ratio );
$h = intval( $current_height * $ratio );
as it was in v3.6.
It’s a hack rather than fix, so do it at your own risk.
