Tyler Cowen quotes Garett Jones:

The key to controlling spending is permitting more earmarks (sic).

Tyler asks us to guess Garett’s model.

One model would be this:

1. Each Congressperson wants to use government spending to buy enough votes to be re-elected.

2. Targeted spending (earmarks) buys more votes per dollar than other spending.

3. Therefore, for every dollar of earmarks you take away, the Congressperson is going to have to add more than a dollar of other spending in order to buy the required number of votes.

This is not the only model, but it is a relatively simple one that gets the result.