As the parent said the cost of that 1GB is not really the disk space but the expected transaction cost in terms of servers. The number of bytes shown as used is not even based on any actual disk usage (this would be complicated with table structure, overhead, indexes and fragmentation). For most tables they use a formula of 2KB per record - it doesn't matter if it's an contact record which is probably stuffed with much more then 2KB, or a very simple custom sales record containing a name and a dollar amount. There are a few special tables that are treated at 512 bytes per record, like the table containing chatter updates (Salesforce.com's social media like notifications). Taken all together this means that the "1GB" of data allowance is really 250,000+ records, depending on how much is chatter vs. actual records and not anything related to disk space. It's just easier to explain it as 1GB to a management person rather then as a complex relationship between records, transactions and indexes.