It's not all that ready to understand what you are saying you found that's 25GB, but usually it's not packages that will be taking up your disk space.
You want a way to look over your directory structure to see where the storage space us being used. You can't install packages for GUI representation, so that presumably means using command line stuff that will already be present. Try this command which will list the biggest files and directories in order by size:
du -max / | grep -P '^\d{3}' | sort -th
The numbers are the size in megabytes.
Hopefully you'll spot some big stuff that you can do without.