This is the difference between a small whippet and a whippet of say 21"+ the standard whippet if fast but the longer back on a larger dog creates a longer stride therefore faster, but loses out in a turn to a smaller whippet, a smaller whippet can turn much more sharply and hold more speed in a turn.
For me I have two 18" whippets both bitches, one from coursing lines and one from race, now given that there both the same size would expect the catching capabilities of both dogs to be the same, but there not the race bred dog is much quicker in a straight line and will put a turn in its quarry