Then why do you think it is that the horse racing "market" had taken this into account by shrinking the size of jockeys, yet IRL racing has not? Is it just a market imperfection that is only being addressed now? If it's such a big difference, then I am surprised that IRL drivers haven't become small over the years.
A horse has only one horsepower, so the extra weight is proportionally a large factor. A 1525 pound car with 6 or 7 hundred horsepower is not so proportionally affected.