When you make a jail there's all sorts of stuff in there that you don't need or can't use from within a jail. Here's a list of files and directories you can probably remove. Edit to taste.
Using this list is entirely at your own risk! Although I run tons of jails without these files, I may have goofed up somewhere. Also note that a normal (non-jail) FreeBSD system won't boot without these files, so don't accidentally delete these in your root system. To remove the files use a command something like this:
# while read file; do rm -rfv /path/to/jail$file; done < jail_remove.txt
If you find other files or problems with this list, please let me know.