That's surprising to me. I didn't expect financial transactions to create that much data. Also wouldn't most of the data be archived and only rarely be accessed?
Financial transactions tend to create a lot of data for the logging and recoverability they require, and the logging isn't terribly interesting.
The trick is realizing that things can and do fail in any way for any reason, and being able to automatically recover from any point in that process. Our (WePay's) system currently requires, at an absolute minimum, seven inserts across six tables, though more commonly 9-10 rows across seven, plus updates on several others, and significantly more if we can't authorize the payment on the first attempt (e.g., mistyped ZIP code on your billing address). Only one of those tables is for logging (account_history, which is functionally identical to a check ledger).
I've completely ignored both the logging of state changes (largely redundant at this point, it was more for early debugging a couple years ago, but still sometimes useful for figuring out why something stalled) and recording of fraud-detection data which can easily be a hundred rows, albeit very small ones (80 bytes or so). It's also interesting to look at what updates need to be performed in a transaction and which ones don't, although that's of course irrelevant to the actual amount of data produced.
That doesn't account for the product side of payments at all - recording contents of shopping carts, donations, ticket purchases, etc. That's at best one stateless row, but use your imagination about the data layout for various money-collecting tools.
Archival is interesting and something I'm sure we'll look at more in the future, but right now it would tend to create more problems than it solves -- we have automated data integrity checks running multiple times per day to ensure nothing is out of line, and kicking data out to an archive somewhere would complicate that significantly. We also of course don't have nearly as much data as PayPal being significantly newer, so it's less of a problem.
conflicts/row locks being one of the more common, but that's easy to deal with. It's when an external call to a processor dies halfway through that things get tricky.
In some cases they store more data than just transaction information. Some people are using PayPal to manage their entire shopping cart. Meaning, that they store basic item information for orders.
