Not sure there is much you can do, I admittedly have an odd body type and am generally wary when I see "athletic fit". I am more a contact sport athletic and less a marathon runner athletic. I routinely have this issue with different brands. When Kuiu and Sitka first came on the market I had similar issues with their apparel. Waist fit fine, but the legs and seat were pretty tight. Over the years something must have changed because now there is definitely more room in the legs and seat area, especially for layering.
I think I could get away with wearing the All Clima pants without any base layers, but they would be pretty snug if I put even a thin base layer on underneath. No worries, just might not work out for me personally. Thanks for making gear that is designed and assembled here in the US.
Edit: Do apparel companies have a "seat and thigh" fit measurement guide? Some companies have a general measurement guideline around the waist, arms, chest, etc. Would be interesting to have a measurement around the thickest part of your thigh or something and see how that correlates. But then again, it could just be me.