I usually take a look at certain questions on both Stack Overflow and Mathematics Stack Exchange, and what's interesting is the vote counts. They usually have around 0 - 2 votes. Now I go to meta, I usually see 4 votes or more. I'm curious why this is so.
People who answer a lot of questions spend a lot of time on the site. They get invested in it and eventually are likely to come across both this meta and their site's meta.
Once you know how the sites work, because you've used them a lot you can start helping other people via these meta sites and unlike main sites where the scope might be very broad e.g. on Stack Overflow there are thousands of programming languages and nobody knows them all, on meta the scope is just about the site itself (or for this meta all sites).
So the people frequenting meta are likely to
- have a high enough reputation to both upvote and downvote
- be able to understand and be interested in all the questions
- be able to vote on most of the site's questions and answers if they wish because the number of questions per day is a small fraction of that of the main site's
Short answer: Fewer questions means more eyeballs per question (a question in a high-volume tag like Stack Overflow's Android tag can easily be seen by a couple dozen people and then disappear with the other million questions in that tag), and the people looking at them are often power users confident in whether answers are correct, so they tend to vote on them more.
There is one notable exception: posts that make it into Hot Network Questions can get quite a lot of votes, paralleled only by Meta posts that are featured. Basically, it's the same principle: if more people see it, it gets more votes.