There software is not on all phones so the total number of SMS messages is irrelevant. Assuming the average text message are 80 charters and they can be compressed 75% then 10GB could be 500 million messages a day and 180billion / year which would be 10% of the total messages in a year. Or with enough meta data 10GB could represent a tiny fraction of that. So, IMO the real question is what information they actually collect not how much data they are storing.