When a file is deleted, the space used on the disk is not reclaimed until the file is truly erased. The trash (recycle bin on Windows) is actually a hidden folder located in each hard drive. To completely delete the file, another step must be performed.
Files, when deleted, are sent to the "Trash". To remove the file and reclaim the space, click the trash icon. A Finder menu will appear with an "erase" button in the upper right. Click that button to erase everything in the trash.
Files are sent to the "Recycle Bin" when they are deleted from a Windows machine. To empty this folder and reclaim the space, right-click its icon, and left-click "Empty Recycle Bin" on the menu that appears.