Did you get someone with an Osprey back measurer to check your torso length? It's key if you want to adjust heavier loads appropriately, and the inch or so too big really can make a difference when you get it loaded up. I'm not much taller than you (5'4"), and my torso is a S. I know that when I have the M packs on, it just doesn't work well for me when they're heavy (I don't notice so much if they're not heavy, so make sure wherever you go to try out packs that you load them up well--your gear will probably be better than their sandbags for trying stuff out, although that could simulate some water weight for you). If you truly are an XS, you probably want a pack that fits you. Size-wise though, you are looking at two different packs (55 litres vs. 70 litres), so you want to keep that in mind when making comparisons. Oh, and it looks like the Xenon replaced the Luna this spring, so either you'll get a good deal on a Luna 'cause it's on it's way out, or you may want to see what improvements they made to it with the creation of the Xenon. I have a Luna 90 from 2001, when they first created it, and I love it still, but I bet there are all kinds of good improvements and updates. Not sure if you're familiar with the "hip-belt, shoulder straps, load adjusters" method/order of adjusting straps, but that's key for a good fit too. Hope that helps! I'm in the market for a smaller personal pack myself right now and it can be time-consuming to make sure you truly get what you want. In the end though, I've found the only way to know for sure is to get it out and use it, so good luck!