There are thinner options, but those options do exist in both disciplines. The thinnest mouth that I have ever used (and likely will ever use) was a 3/8 inch diameter. It is only slightly smaller than my regular snaffle bit. BUT, I don't ride on contact either so the potential harshness hardly ever comes into play.
Where an English rider is expected to keep contact with the horse's mouth, which I am sure you are accustomed to seeing so I won't bother with a picture.
A western rider who knows how to ride well and is riding a good horse will ride with a fairly significant amount of slack in their reins, no matter the gait, like this
Even when cueing for a turn, the bit should not be effected.
Here, especially, you can see the bend in her body even though the reins are still slack
As for the curb bit you are describing with the flat metal looking thing with a roller, are you talking about a spade bit?
If that is what you're asking about, then that is an exceptionally advanced piece of equipment. It sometimes takes a horseman half a lifetime to learn to use one properly and each proper "bridle horse" takes years to train and prepare them to carry and respond to such a bit.