I haven't had the medium ruck, but the large is adjustable and most people dont realize it. Check to see if you can adjust the fit to your torso length, it will save your back and shoulders in the long run.
Most people wear rucks and packs wrong. Your shoulders should not support the weight, your waist should. Shoulder straps should just assist with stability and position. There are multiple videos and articles on how to properly size and wear a pack, especially in the backpacking crowd.
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