Unfortunately, there isn’t a lot you can do about this. Google exercises a lot of control over their ad sizes and don’t really allow you to change them. The only real option is to choose a smaller ad size (such as a 468×60 banner) to display beneath your posts, or to go a single column display in Graphene.
The other option would be to use an action hook to display your leaderboard at the top and bottom of the page. A bit of work, but not entirely difficult.
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