http://horse-science.com/Distances.htm
If you want to get really confused, click the answer to question #2 on this web site. Contends there is a speed impact on a wider horse but no distance penalty impact due to the tightness of a turn. I think this means that the tighter the turn the faster the wide horse has to run to maintain its position.
Also,says that the distance penalty of a wide turn is 15 feet per path per turn regardless of the turn tightness....the 15 feet is more than is commonly accepted(I think).