Bonus units are from delegation or for participating in the management team. We have a writers group that gets bonus units for contributing to our regular updates, for example.
The second question I believe you are talking about your own vote value on Steem, not your SBI vote size? Your votable value is determined by your SP, so you increase that to increase your voting value. (curation/author rewards, investing and powering up, or leasing delegation through dlease.io or delegationhub)