I've noticed certain files which are only, say, 12% downloaded, create really big files in the incomplete folder - like 300MB.

Is this normal ?

My hard drive space seemed to be disappearing, and I'm not sure it was just the big incomplete files.

Anyone have any explanation or suggestion ?

Thanks

Sounds normal. If you download a 300MB file from multiple sources, sometimes you get one of the chunks from the end of the file sooner, and the middle pieces later.

Virtual memory can eat up lots of disk space too. Restart the machine and see if your disk space jumps up. Virtual memory disk space is also 'restarted' after a shutdown.