I guess the glib answer is that the big thing is specifically designed to avoid disrupting those signals, and the small thing is designed to disrupt them as muc

I guess the glib answer is that the big thing is specifically designed to avoid disrupting those signals, and the small thing is designed to disrupt them as much as possible. A tiny radar reflector has the cross section of a huge object, because it intentionally has right angles that send signals back to their source without scattering them in other directions (as much as is possible). Passive radar still requires the object to reflect signals, it just doesn't require the signals to be reflected back to the source of the signal. Consider a monitor with an anti-reflective coating (so lets take a non glossy monitor). It is effective at reducing the reflection of the window behind you, it uses a thin film to cause destructive interference so that the light coming in the window not reflected, and you see a greenish-purplish image of the window instead of getting a bright reflection. However, if you take a flashhlight and shine it down on the monitor from near the top of the monitor, you will easily see the glare from the flashlight as it illuminates the monitor. I think this is a good analogy for active vs passive radar. The F-117 was built to avoid right angles, this is like the light from the window behind you. The F-117 won't send radar back in the same direction it came from, so active radars are thwarted. However, if it is being 'lit up' from somewhere else, it is not as effective since by not sending radar back to where it came from it sends it in other directions instead. However, it also has a coating to just straight up absorb radar. The F-35 is much more reliant on such coatings, and so it should scatter very little EM radiation in those frequencies, regardless of whether the detector is in the same location as the transmitter.