In the case of a 2 series or a 3 series - all the legs are thinner in diameter and the top section presents a difference - but not extending the bottom section would improve performance. If you are going to use a tripod for a heavy lens at or beyond its capacity, not extending the thinnest legs to their maximum should improve performance.
The 3 section tripods benefit from not using the thinnest section (of a 4 section) to achieve their height but offset that with longer tube length which might suppress a little less vibration. The XLS is essentially a 3 section with an added leg section. Given the choice, the more I can decrease the use of the bottom leg section the better my performance will be. Now an inch or two probably makes an insignificant difference, but in general using larger tubes provide more support and less vibration.
I've wrestled with the idea of an XLS - which I need for hills and streams - vs. a 5 series for my 600 f/4. I know the 3 series LS is a compromise, but it works well for travel. My guess is the XLS and Series 5 would each get 10-15% of my images during the year. A 5 series would be perfect for my 600 f/4 - but does nothing for the extra length needed. I really hate to need three tripods so its all about compromises. The Series 5 is tops on my list right now.