One of my office developer uploaded videos to svn repository.Few days later he removed those videos but svn repository still showing very large sige.How could i decrease svn rep
Your question is answered in Apache Subversion FAQ:
"How do I completely remove a file from the repository's history?"
Apache Subversion (and other version control systems) is designed to keep all the history. Removing files from repo history is commonly considered as a complicated task and not really a good thing. That's not the task which you can easily accomplish.
Just in case someone browses this topic several years later: a file or directory can be removed from subversion - you make dump, you filter dump, you restore filtered dump. For more details see https://stackoverflow.com/a/6025750/1727132
SVN keeps revisions if you delete them. See http://www.svnforum.org/threads/35631-permanently-delete-file to delete the files permanently.