What contributes to the transaction size on a MySQL UPDATE?
The transaction size is not just based on the amount of data you're actually change, rather the whole state of the row before you made the change is preserved in case of a rollback and therefore the entire row is essentially copied. From further research it sounds like as much as even the entire page of rows is copied to the rollback segment, so depending on how many pages contain the rows you're updating, it could easily be a lot more data generated in the transaction then what you're actually updating.