But a bare-bottomed spanking is generally seen as more humiliating than, say, being flogged on the upper back while still wearing pants. Doesn't that indicate that it's something other than the pain that's causing the humiliation? (In the case of bare-bottomed spankings, I'd say it has a lot to do with being treated like a child. Spankings are what we do to naughty children; flogging is more of an adult punishment and therefore less humiliating.)
Now... being powerless or overpowered can be very humiliating, and bondage and pain infliction definitely play into that. But I don't think that means the acts are inherently humiliating - to some people, the powerlessness aspect doesn't feel applicable because they know it's only happening by their consent, and they might even see the pain as a chance to show off how tough they are by taking it - turning what would be humiliating for some people into an source of pride, even.