These files are not complete.

The reason the filesize shows the size of the complete file is because most if not all P2P programs download random chunks of a file depending on what's available. These pieces then get put in the right place in the file when written to the harddisk. Some programs give you the choice of whether to allocate the space before download or not, and doing this is recommended. Limewire does this for you and you have no choice in the matter, which is actually a good thing.

I know that if you have space problems, this can be a bit annoying at times, but it's still for the best. This way your harddisk won't become as fragmented as it would otherwise be, and keeps performance as high as possible.