Jupiter was formerly twice its current size, had a much stronger magnetic field

A prepublished version of the paper is available on arxiv [1]

[1] https://arxiv.org/abs/2505.12652

The mass did not differ much from the present mass, but the planet was less dense and with a more rapid rotation.

It was hotter, a lot hotter.

Like stars, radius for a gas giant is increased by heat, and decreased by increased mass.

These two factors are rarely completely independent, of course, so it gets complicated. Especially in a star where masses are large enough to result in densities sufficient to cause fusion - and large releases of heat, which then cause decreased density, etc.

But all other factors being constant, the volume of a gas increases (and density decreases) as temperature increases.

See page 6 and the first couple paragraphs of page 7 in the paper for a breakdown.

Eventually Jupiter will cool enough it will be a small fraction of it’s current size, assuming that our understanding is correct and it doesn’t have enough mass to meaningfully result in fusion regardless of how dense it gets. [https://www.pas.rochester.edu/~blackman/ast104/jinterior.htm...]

In theory, it will even eventually cool to the point all those clouds and atmosphere are liquid (or even solid!) gas oceans. That is going to take awhile.

Probably contracted as it's mostly gas.

Not larger mass. Simply, was less dense. In layman terms, and if I understand correctly, was the result of the interactions of Jupiter, Jupiter's magnetosphere and Jupiter's circumplanetary disk.