Index: wp-includes/media.php
===================================================================
--- wp-includes/media.php	(revision 12644)
+++ wp-includes/media.php	(working copy)
@@ -484,7 +484,8 @@
 			ksort($areas);
 			foreach ( $areas as $_size ) {
 				$data = $imagedata['sizes'][$_size];
-				if ( $data['width'] >= $size[0] || $data['height'] >= $size[1] ) {
+				// find a size where *both* width and height are bigger or equal to the required
+				if ( $data['width'] >= $size[0] && $data['height'] >= $size[1] ) {
 					$file = $data['file'];
 					list($width, $height) = image_constrain_size_for_editor( $data['width'], $data['height'], $size );
 					return compact( 'file', 'width', 'height' );