When copying data from one drive to another is it common to have slightly different file sizes or should they be exactly the same?

If I make a copy of a folder (or a whole directory) to another drive and want to make sure that no data was lost, is it acceptable if the file sizes displayed are slightly different? Could this just be a product of the data being on a different drive and the size being calculated a little differently. I am talking about the folders being off by a a small amount- like 300 bytes and the whole volume being off by a few kilobytes