I've been trying to work out a few calculations for a planet I am working on and hit a bit of a roadblock. The planet in it's current state is 1.35x the size of Earth, but in order to get a density similar to that of Earth's so that it could support a stable magnetic field I made the density 20% more dense than that of Earth's and 3x as massive. This has come with a side effect of 60% higher gravity which isn't idea for the world I am trying to create, specifically the size of life. I am aiming to make gigantism more common, although more in line with Earth's prehistoric periods or the planet Pandora's megafauna, but 60% more gravity seems to prevent that from happening.
The other solution would be to make the planet 80% as dense as Earth, making it 2x as massive and only 8% more gravity than Earth's. The issue I am worrying about is the potential for a magnetic field strong enough to hold in the atmosphere and protect the planet from potential solar radiation. Would this be possible? A planet 30% larger and twice as massive as Earth to be 80% as dense and still have a dense core like Earths?