Yes, that's exactly it. Steem has a rule where your right to add data to the database is proportional to your Steem Power. Votes are the most overhead-laden transaction, they have all the same data in the container it is wrapped in, but only the URL for the post/comment, and the amount of VESTS that are given in the vote. Thus it can become excessively spammy very easily compared to the stake of the user.