If you do not prune and / or removed unsed box you can quiet easily see yourself waisting space.
Case in point.
bclark@bclark:~$ du -sh .vagrant.d/boxes/
5.7G .vagrant.d/boxes/
If you want to see what boxes you have:
vagrant box list
i.e.
bclark@bclark:~$ vagrant box list
debian/stretch64 (virtualbox, 9.5.0)
debian/stretch64 (virtualbox, 9.7.0)
debian/stretch64 (virtualbox, 9.8.0)
debian/stretch64 (virtualbox, 9.9.0)
ubuntu/bionic64 (virtualbox, 20190212.1.0)
ubuntu/trusty64 (virtualbox, 20180814.0.0)
ubuntu/xenial64 (virtualbox, 20190325.0.0)
To clean up you can use:
vagrant box remove debian/stretch64 --box-version 9.5.0
HTH
Brent