In order to sustainably offer the flexibility of paying-per-project, the upload quota is limited to each gallery. You can add more media to a gallery at any time, but any unused quota cannot be transferred to other galleries (unless it's above 5GB).
Quota Credit for unused quota over 5GB:
If your gallery has over 5GB of unused quota, you may be eligible for quota credit to be used toward other projects. For example, if you have a 20GB gallery and only used 14.3GB of quota (5.7GB unused), we can credit you 5GB of quota toward another project.