What makes molten chocolate so much more filling?

Having done something very similar the other day, I think the banana accounts for more than you think (you don't say how big your slices were, but 3 big slices could easily add up to the whole very small banana I coated in chocolate left over from dipping these biscotti). The butter will make a tiny bit of difference unless you used loads. I don't add any when dipping, and chocolate is mostly fat anyway. Fibre in the banana is filling, and fat from the chocolate is satisfying, so the combination makes you really feel like you've eaten.

Another factor is that dipping cookies uses up surprisingly little chocolate, and chocolate looks like less when liquid so you may well have had more chocolate than you thought. By the time you dipped the banana it might have thickened a little so you could scoop a nice thick layer.

You didn't by any chance have a few mishapen/broken cookies in the batch that needed "testing", did you?

I think there are a couple factors at work.

One thing is, hot food tends to feel more filling or satisfying than cold food. I recall reading this effect is less pronounced in some cultures (like japan) where food is commonly eaten at room temperature, so there's likely a jolt of psychology and culture involved.

Another thing is, the fat, the richness, will tend to make the chocolate feel satisfying. You mention that you don't think the volume of butter added is the key, but I think it may help, along with the availability that comes from the chocolate being melted. Just like a food tastes saltier with salt sprinkled on the surface rather than intermixed, the availability to the tongue makes a difference in how something tastes. Melted chocolate is very available to the taste buds, as it coats the tongue and mouth, and warmer foods tend to be more active, again being more available to the taste buds. A chocolate bar is harder, cooler, and it would be chewed - in chunks which are only just melting, the tastes are less available to the tongue. I suspect you would find a difference in how satisfying the same bite of chocolate is chewed or kept in your mouth long enough to melt. Even so, already molten chocolate will feel richer than melted-in-your-mouth, since the texture of the latter would probably seem a bit thinner (from intermixing with saliva as it takes the time to melt), and the semisolid thickness gives, again, an impression of richness and substance which is quite satisfying.

I'd guess there is one more factor, and that's the texture. Because molten chocolate is rich and thick, the it coats the tongue pretty thickly in a way that suggests a need for something to cut the richness, usually something to drink - the same is true of most foods with thick clingy textures, like peanut butter, etc. The intensity of flavor (especially concentration of sugar and salt) may contribute, it would tend to pull moisture out of the mouth (a bit, just a tendency). This sounds ridiculous when I'm trying to explain, but it feels right when trying it - anything thick and sticky (especially if also warm, or rich) feels better followed by something (thin/cool/sharp) to cut the texture and cleanse the palate. And wanting a break every bite makes it seem like more has been eaten, or even too much has been eaten, much faster.

To dig a bit into your experience, I'd guess that possibly taking the volume of banana into account, you had enough food to satisfy your belly. And while you likely could have eaten more (up to that whole chocolate bar you mention) without issue, the desire that would draw you to finish the bar would be more want, or satisfying the tongue. Since the molten chocolate feels rich, thick, satisfying the tongue, it felt like enough once your hunger was satisfied, instead of like wanting more.