They’re making the (possibly false) assumption that density will stay the same, so size must grow with weight.
Of course, it’s equally likely that size will remain constant. It changes or it doesn’t -> 50/50 odds. Until the point it all collapses into a black hole.
Endless Sky is an amazing one. I’ve put tons of hours into it. It’s a top down 2d space trading/fighting game, very similar to Escape Velocity if you’ve ever played that game.