We all know the tale of Jack and the Beanstalk--a farmboy sells his cow for some "magic beans", which in turn grow into a mountainously tall stalk that led him to the land of a giant. Now, scientifically speaking, the only way for Jack to meet the giant is if the beanstalk led him to a habitable world with lower gravity.
Which was the sort of thing that "Jack", a member of the mad scientist illegal terraforming community, wanted to take advantage of when the real scientific community discovered that one of the alternate Earths has an iron-rich core the exact same size as ours is, but hidden beneath a smaller crust, which compresses the mantle. This combination of smaller crust and same-size core means that the planet would be warm enough to support liquid water, therefore life. For him, it is the perfect place to turn the story of "Jack and the Beanstalk" into an actual ecosystem, complete with smuggled stores of fabacean seeds, or "beans", just to see if the lower gravity would be enough to turn these small herbs into giant plants that anyone named "Jack" could actually climb up.
But here is the thing--if the crustal diameter is too small, therefore compressing the mantle too tightly, then the crust would liquefy, turning the surface into a volcanic hellscape. So in an alternate Earth where the core is the same size, how much smaller would the crustal diameter be for this world to be livable instead of hell?