Adding a volume to your Droplet doesn’t increase its main disk. Instead, it adds on a second disk as a second partition. To expand the Droplet’s main disk, you need to resize the Droplet to the next plan.
If you just need additional storage, you can either add a new volume to the Droplet and move some files to that volume, or configure all the logs in
/var/log/ to save directly to the new volume.