That Pu238 pellet is surrounded by ash, I think they buried it in ash to hold the heat in and then uncovered it so you could see the glow. In this case it is driven by heat transport so the bigger of a mass you have the more the ratio of volume to area and the more it heats up.
That's very different scaling from this kind of thing
https://en.wikipedia.org/wiki/File:Plutonium_pellet.jpg