How big of an advantage can it be? In horse racing, it is a big advantage, therefore the jockeys are all elves. If it was such a big deal in IRL, wouldn't the racers over time all become little elves too? Clearly the individual teams think that it is not a very important factor.
IRL does not adjust for driver weight.
In fact, most of the world's major automobile racing series do make weight calculations based on car minimum weight AND weight of driver; the FIA Formula One series is a good example of this. That way, every car's weight is carefully adjusted to weigh pretty much the same, eliminating the advantages of lighter drivers.
In short, if IRL were to impose a driver weight equivalency rule, Danica Patrick's car could be saddled with as much as 60 pounds of ballast weight. That could slow her down quite a bit, especially in light of the fact that an IRL racer has a minimum weight of 1,525 pounds.