Radar cross section is a function of the signal backscatter density in the direction of the radar receiver. That is, it's all about how much signal is bounced

Radar cross section is a function of the signal backscatter density in the direction of the radar receiver. That is, it's all about how much signal is bounced back. Small things that bounce a lot of the signal back (see: radar reflector) have very large cross sections. Similarly, if an object is designed to absorb or reflect in a direction away from the receiver can have a small cross section. The F-117 is "radar stealthy" because all of those funky, flat facets all over it reflect small parts of the radar signal in odd directions so that very little is reflected directly back in the direction of the emitter (which is one reason why they aren't in service any more...air defense folks figured out that multiple receivers spaced out plus a little signal processing could tell you where an F-117 is).