You could continue to allow 5mb for each subdomain, but wait until say, 25mb for the sum(*.domain.tld) before prompting. For example, this would allow {www,mail

You could continue to allow 5mb for each subdomain, but wait until say, 25mb for the sum(*.domain.tld) before prompting. For example, this would allow {www,mail,docs,plus,???}.google.com to continue as they are now until google.com managed to rack up 25mb worth of data. After which point the user might want to know they're holding on to a fair bit of data for one particular site. Then again, prompting is really annoying, and most people just click "okay" and without comprehending.