Raspbian is a great distribution for the Raspberry Pi. However, by default it installs a lot of big packages meant for schools and education. This is not great for many users who tend to install it on a SD card which has limited space and use it as a headless computer. I was surprised to see little space left on a 4GB SD card after installing Raspbian.
I was able to find the biggest packages by install size using Aptitude, as described here.
The biggest installed packages turned out to be:
wolfram-engine 460 MB oracle-java8-jdk 186 MB pypy-upstream 57 MB scratch 46 MB freepats 34 MB
I was quite surprised to see the gigantic Wolfram and Oracle's version of Java 8 (and not OpenJDK) being installed by default in Raspbian!
After removing the big packages that you don't need, you can gain back a lot of space :)
Tried with: Raspbian 7