Voting power is based upon how much Steem Power (aka vesting STEEM) an account holds AND how much voting an account has done recently.
Voting power is based upon how much Steem Power (aka vesting STEEM) an account holds as well as how much voting an account has done recently. The Steem network attempts to approximate your average voting rate such that your voting power on any given post is approximately:
SteemPower / VotesPerDay
You can increase your voting power by posting and voting.
Full explanation: https://steemit.com/steem/@tuck-fheman/how-is-voting-power-determined-in-steem