I found this logic in "wp-admin/inline-uploading.php":
if ( $imagedata['width'] * $imagedata['height'] < 3 * 1024 * 1024 )
// consider making a thumbnail
Now, I suppose the idea here might be to avoid using too much memory, but it also seems to me that the more pixels a picture has, the more desperately it needs a thumbnail. I would be interested to learn about the thinking behind this limit and the likely consequences of commenting it out.