Im looking at diameter and how you touch the horse with it. With spurs you us pressure and roll the spur into using however much is required to get a response. If you ever have access to multiple spurs lay them along your thigh like you would on a horses side, and roll them in. See how it feels. I have noticed that I like the western spurs more, and so do the horses. Its like laying on a bed of nails, would you rather lay on one nail or a couple hundred? I prefer the couple hundred. I've had some horses calmly respond to the large rowel western spurs versus jumping away from the english ones. So In a sense of surface area vs length = severity.
This spur would hurt the least :
^^ Big with lots of very short spikes that wouldn't feel too much
^^Small, but short enough to not be that much of an issue.
This ones in the middle :
^^ I use this for western work. It rolls nicely off the sides, and has a wider rowel than an english spur, but is also smaller, and long off the boot, so any harsh riding or kicking could cause pain.
^^The length makes this more sever
Then a level harsher :
This is because of a flat edge its diameter and length.
And this one as the harshest of all : (I think its english...but you can find spurs like this in english AND western. I have yet to see an english one like this in person but have seen a few western, but overall it is uncommon)
I hope I got these all organized right. But its all a look at HOW you're using it. I know some riders who could ride in that last spurs and have the right technique to use it without hurting the horse at all...but it requires such precision that I would not use it. I use no spurs for english, use the squeeze, cluck, smack system. Western I use spurs for turning around the barrel, nothing to do with speed or a lack of respect for me heel in forward momentum.
This is the physics I understand it to be, If I'm wrong feel free to correct me.