Now that you mention it, I was somewhat curious. I ran the following simple query on both Super User and Stack Overflow to see how many users had edit permission (>= 2000 reputation):
SELECT
COUNT(*)
FROM
Users
WHERE
Users.Reputation >= 2000
Which returns 488 for Super User, and 18890 for Stack Overflow. As a percentage of total users (i.e. commenting out the WHERE clause in the above query), and repeating the query for Android SE while we're at it, we get:
Super User: ( 488 / 136571) x 100% = 0.357%
Stack Overflow: (18890 / 1582958) x 100% = 1.193%
Android SE: ( 32 / 19260) x 100% = 0.166%
Given these numbers, it appears as though Super User has roughly 3.3 times less editors as a percentage of all users on the site as compared to Stack Overflow.
I also thought it would be cool to evaluate the above based on number of questions; so below are two values, the first is total questions + answers divided by users, the second is questions + answers divided by number of users w/ edit permissions:
Super User: (151k + 265k) / 136571 ~= 3.1 Posts/User
(151k + 265k) / 488 ~= 852 Posts/User w/ Edit Perms
Stack Overflow: (4.2m + 8m) / 1582958 ~= 7.7 Posts/User
(4.2m + 8m) / 18890 ~= 646 Posts/User w/ Edit Perms
Android SE: (11k + 16k) / 19260 ~= 1.4 Posts/User
(11k + 16k) / 32 ~= 844 Posts/User w/ Edit Perms
Thus, it looks as though Super User has more posts per user than Android.SE, with a similar number of posts per editors. This may indicate a higher volume per unit time on Super User.
Stack Overflow has a significantly higher number of posts per user, but also has a proportionally higher number of editors, obtaining the lowest number of posts per editors.