Hi.
The vault size shown is the free space of the partition where your vault is stored. If you are using WebDAV as volume type always the free space of your system partition is shown (see here).
But this is not the space you are using. It’s the available space.
Cryptomator vaults do only consume the space of the files in the vault. It’s a file based encryption. (In opposite of Trueceypt which is a container based encryption)
So if you do not want your vault to exceed 100 GB, you just have to stop putting files in it when the files in your vaults reach 100 GB
There’s to function to limit the vault size or show a warning when a size is reached.