The Heritage has the slimmer primary due to the slimmer rear wheel/tire.
It's definately wider than my '16 RK and '09 Dyna. I sat on an '18 Fat Boy and it didn't feel wider to me, but that could be because the bike sits about an inch lower.
I think it’s only on the wide tire bikes like the Fat Boy and Breakout but I am not certain of this.
Nope... Even though the Heritage has the narrow primary, it's still up against my left calf. I'm 5'7" and wear 30" inseam pants. Noticed it both times I sat on one at the dealer (haven't got around to test driving one yet). Last time I went over and sat on a new RK for comparison. No issues with the primary on the RK.
My wife sat on one and complained about the primary pushing her leg out. She has a '16 Fat Boy Low right now that fits her short legs well. She did like the new kickstand placement though as she struggles with that on the '16.