Some of it might be down to perspective. Sometimes bodies such as the Sun and the Moon can appear larger or smaller than they actually are depedning on what else you are looking at.
Well, yeah, but in this case the whole point is that Jupiter and Saturn are supposedly the exact same distance as the moon. According to the scale presented here, I'm way less impressed with how big those planets are.
Though part of me does wonder if the time of year and the sky might have played a role. The moon was presented as being quite tiny, yet there are definitely days throughout the year when the moon looks HUGE because of the way the light is bouncing around in the atmosphere.