How to recalculate uploaded files' sizes in KB and update the values in the database?
In Drupal 7 managed files have their filesize property updated upon saving in the file_save() function. So you can just load/save each resized files to have it updated in the DB accordingly.

//@TODO: Get list of $fids corresponding to resized-files
foreach ($fids as $fid) {
  $file = file_load($fid);
  // $file-&gt;filesize reflects old size
  file_save($file);
  // $file-&gt;filesize now reflects new size
}