The Lego planet does not necessarily need to be smaller than earth. Remember, it's made of Lego bricks which are not 100% solid, they have pockets of air inside

The Lego planet does not necessarily need to be smaller than earth. Remember, it's made of Lego bricks which are not 100% solid, they have pockets of air inside them when connected to other bricks. You need to take into account the mass ratio of a Lego brick to be able to correctly correspond the mass to a radius.