So when Chewie, Finn, and Han are looking for Rey and running around in Starkiller base, which is presumably enormous, and of which there is zero indication that they’ve memorized the complete blueprint/layout of, they suddenly spot her across this huge chasm. SECONDS LATER, they are IN THE SAME ROOM WITH HER. THE BASE IS THE SIZE OF A SMALL CONTINENT.
Look at this, Finn’s pointing at the thermal oscillator here. In the movie, that looked about 300-500 metres wide from all the shots we see from inside it and the close up shots from the aerial strikes by the X Wings.
So therefore if the scale shown in this hologram is supposed to be accurate, Starkiller base is a VERY VERY small planet, many times smaller than The Moon, with probably a 40km radius, making the internal base small enough to see each other by chance quite likely