i think i learned in school that atoms and stuff are made up of mostly space, and if you were to "vibrate" at the right frequency you would be able to pass solid matter through solid matter.

now, if they used the mass of planets as raw building material, the planets are pretty dense. but the city is buildings, tunnels, and stuff like that. i think maybe only the megastructure walls seem to be solid and dense. so if you were to extract raw material from something like Jupiter which has a whole bunch of mass and then use it be build humanoid structures that are a lot of open space, i think you could stretch the mass a lot further, distance-wise.

you have to take into account that the biggest planets are mostly made out of gas and liquids or liquid gasses. only a relative small amount of them is solid matter.