The idea being debated
Some people say: "Animals with simpler brains are less sentient." And from that they conclude there should be a ladder of importance: humans at the top, then other primates, then other mammals, and so on.
Sentience doesn't come in degrees
If "sentience" means the ability to have experiences, then it doesn't make sense to talk about degrees. Either someone has experiences or they don't. It's like being on or off.
What comes in degrees is intensity
What can vary is how strong, good, or bad an experience is. This has a technical name: valence. A pain can be mild or unbearable.
Does a bigger brain mean feeling more?
Not necessarily. Different beings might feel the same thing with different intensity. But that does not mean one kind of animal always feels more strongly than another, in every situation.
Sometimes it might be the opposite. Pain exists to make an animal get away from danger. In an animal that's less able to reason, pain may need to be more intense to work, because it can't think "this is dangerous" in any other way.
In short
- Either you have experiences or you don't: sentience doesn't come in degrees.
- What varies is how intense experiences are.
- A simpler brain doesn't mean feeling less.