Space is calculated by the size of the images you upload to your Bubbles projects. You can get additional space by subscribing to a Pro workspace, which comes with 5 GB. You still get to keep the 1 GB of storage in your personal workspace, but the storage space in all your workspaces are kept separate. If you want to add storage space to an existing Pro workspace, you can purchase additional seats, each of which will add 5 GB of storage to that Pro workspace.
Pro workspaces are coming in early 2023.