If I have an image with multiple architectures, quay doesn't compute their total size to the Quota.

Why?
Probably due to a bug when doing the used size:
A multi-arch image is defined using a mediatype = 16 ( application/vnd.docker.distribution.manifest.list.v2+json ) so then the UI can iterate over the different mediatypes listed in the manifest and get the right layer (every layer is an image for an specific arch ). The layers has a right size field but probably the quota is expecting a mediatype = 15 ( or 13 ) to add the image to the uses space.

As we can not know the space used and the quota is not working for a multi-arch repository, it can grows without limit and fill the disk space