Could you give a cite for this? It's counterintuitive to me. I'd think that any internal heating would cause expansion pressure that would tend to increase the size of a brown dwarf.

In fact, this was an early suggestion for the source of the Sun's energy output: continued gravitational contraction, opposed by internal pressure from heating. Unfortunately, that only kept a star shining for a few millennia; and it required a dense sun (iron) even for that. When the Sun was found to be mostly hydrogen, the gravitational heating theory fell apart.

Anyhow, I'd like to understand how an object with several times Jupiter's mass, and greater internal heating, could be smaller and denser than Jupiter. (Assuming similar compositions, of course.)