The box file size doesn't increase by creating a larger disk partition by default; it dynamically resizes to grow as space is used within the VM.  From the Vagrant docs (https://docs.vagrantup.com/v2/boxes/base.html):

"For example, in VirtualBox, you should create a dynamically resizing drive with a large maximum size. This causes the actual footprint of the drive to be small initially, but to dynamically grow towards the max size as disk space is needed, providing the most flexibility for the end user."

I'm not aware of any method for automatically resizing the disk of a base box. See e.g. http://askubuntu.com/questions/317338/how-can-i-increase-disk-size-on-a-vagrant-vm/318711#318711 for a workaround -- but I'd definitely not push that onto e.g. web developers using these VMs, and if I'd have to save my own box image, I'd just create one from scratch in the first place.

Am I missing some method for resizing the image easily on the fly?  If so, that seems to contradict the Vagrant docs.

-Jeff