lots of new hd space after sorting trash bin

So i had the problem where when i deleted files etc they did not go to the trash bin, i just got a message saying this is undo able and if i click yes the files are permanently gone. So i searched the forum to find a thread covering this with a answer to the problem. I used terminal and the command

sudo rm -rf ~/.Trash

I then restarted and now the trash works as it used to, files go there first before being deleted but i seem to have gained about 20gb of free space on my hd?
I cant see that anything is missing yet and i do have time machine back ups so i am not to worried just wondering where all this free space has come from?

cheers

Some applications put hidden cache folders into the Trash once they are finished using the data. I think that Roxio Toast is one of the applications that do this.
Apart from that the system also has a folder where some temporary junk gets stored until you restart the computer.