I think its in relation to the number of ads being displayed. I.e. in the first ad unit there are 4 ads while in the second only 3. I think it automatically adjusts the size to fit the space. I dont know of any methods you could use to control this.
Happens to me all the time, I don't think there is a way of changing this lol If there is, someone please tell me!
it's related with quatitty of ads .there are 5 link on the first ads but there are 3 links on the 2th ads
Bigger fonts means less ad links..........the ad links quantity grows with page impressions by the user, try this if you have less link with bigger fonts reload the page twice or thrice..the adlinks will increase with the fonts getting smaller