Get the scoop on your existing pack by loading it up and trying it. Might as well find out now. As to the Osprey, if you can get to a store and demo it with sandbags, hopefully with a salesperson who can fit you properly, that should tell the tale. IIRC the Atmos series are mid-weight haulers but they've expanded the line since I last looked at them.

Cheers,
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--Rick