You may be just outside the size range for a medium.

Have you used the load lifter straps? They take weight off your shoulders. Or try shrugging your shoulders while cinching down the hip belt.

If that doesn't work, oh well your wife can enjoy it.

Or......maybe you can still return your medium and get the large on sale here with free shipping.

I think it's a great little pack. I have the older model with mesh pockets.
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If you only travel on sunny days you will never reach your destination.*

* May not apply at certain latitudes in Canada and elsewhere.