Okay I just downloaded some songs, like 17 songs which takes only about 50MB (it's shown at the bottom of the playlist on iTunes) but my HDD space has reduced by 1GB!
Clearly something is eating my macbook's HDD space and I am really worried! I've tried using clean my Mac, but the eaten memory has not returned to me! Help!
First of all, there are tons of things that can take up space on a computer. Maybe a security update downloaded in the background, or just using the web browser will cache pictures so a website will load faster next time you use it. Basically, it is normal for space to fluctuate some, and a GB is not a lot these days.
Also, just running CleanMyMac isn't going to magically give you space. When you run it for the first time, tons of space (relatively) will be recovered, and running it again a day later isn't going to recover nearly as much space as the first time.
Think of it this way: Over the course of a week, you can fill up a trash can, and now it smells bad. That was your computer before running CleanMyMac for the first time. When you empty the trash can (in this case run CleanMyMac) there is no more smell coming from the trash can and the air is fresher. If you only put a bit of trash in the can over the course of a day, it isn't smelling bad yet. If you empty it at the end of the day, the air won't magically get fresher.
I think you should get a new, larger hard drive, maybe around 500 GB, because with a hard drive that full you must be constantly deleting things to keep any free space at all and it is just slowing your mac down.