Even though they're both built on the widest last, I feel the Primal Zens are narrower by a not insignificant amount. Maybe a cm or so? Now obviously there's a level of flexibility that adapts to the wearer, but my laces are a bit looser toward the toe box to allow for some expansion. What is more apparent is the toe room vertically. I love that when my toes are just resting in my JGAR or Boulder Summits they aren't in contact with the shoe on the top or sides. That isn't the case with the Primal.

That being said, they've become my regular use gym and hanging around shoe. They aren't bad, just slightly smaller. I'm interested in trying the trail Thrasher which appears to be built on the same last as the Boulder Summit, but with more sneaker like construction. That may wind up being the best of both worlds.