How well do these fit under a helmet? They look like they might stick out and rub on the helmet liner.
They fit just fine under my Shoei X-11. YMMV depending on your ear canals.
One tip though: When I got my custom earplugs made by Al Schibi, he showed me how to cut some foam out of my helmet to make it more comfortable.
This is NOT the EPS that protects your head if it hits the road, this is the open-cell padding around the earholes.
Most helmets have earholes that are WAY too small, mine included. So I removed the liner and cut away some of the foam around the earholes, a little at a time. Huge increase in comfort from not having my ears smashed anymore. Just that tip was worth the price of the custom plugs.

The added benefit is that my EAR headphones fit under the helmet too. There is no decrease in safety related to this mod.
I got my custom earplugs from Al before I had my headphones, so I don't have any before and after stories on that, sorry.