not positive , I own a pair of smart wool leggings , and the Helly Hanson , underwear.. my advice is yes , get the same size , if they're comfortable , they both have a stretchy waist band , and stretch all over .
I have not ever purchased Smartwool leggings. However, I considered the Smartwool before finally choosing the Helly Hansen because after comparing the reviews and the description, I thought they would fit my long legs, athletic thighs and backside better. That said, I have tried on Smartwool tights and found that I needed a L...which is the size I ordered in these leggings. If you look at the specs, I believe the HH are also longer than the Smartwool.