GoLite's pack sizes are based on spinal length, which is roughly but not perfectly correlated to height. Here's the sizing info: http://www.golite.com/images/sizing/packs.gif.
Within each pack size, the pack will fit differently those with different spinal lengths, since the pack size is not adjustable (a feature that would add 1-2 pounds, plus complexity and cost, to its weight). i.e. A Medium Jam2 pack will be longer on a person with a 16" spinal length than someone with a 18" length.
If you have a preference as to where you keep your belt, and if you are on the margins between 2 pack sizes, you might take this into account. For example, I have a 18" spinal length but definitely keep with a Medium because I tend to wear the belt closer to my waist (which I think is common among lightweight backpackers who find that lightweight loads don't sit on the hips as heavy loads do, and in fact they ride up frequently towards the waist). If I wanted the pack more around my hips, I might consider going with a Large.