The rules for Indy are that the car is weighed WITHOUT the driver. Thus giving a bigger advantage to those who weigh less. Especially in fuel mileage.
My experience is that most open-wheel racers are in the 5'6-5'9 category, so there's not much weight difference. But a few come to mind - Dan Gurney and Eddie Cheever and (I think) Jerry Grant - who were over 6', and that probably hurt them a little bit. (I remember when Gurney drove the Ford at LeMans, they had to have a little bubble in the roof of the car to accommodate his helmet - he was too tall for the car.) But back in the 1950s, Jose Froilan Gonzales drove for Ferrari in F1 with considerable success despite the fact that he was an actual fatso, fatter than Foyt.