First it is a very big diffrence between groundbased radars and onboard ditos. And when a B-1 is detected it usually is by EWR-bases etc. Also in the aircraft there is diffrent scanning methods when it comes to air contra ground scanning. But what they probably mean, is that in "larger" they mean they will more likely spot a truck because it reflects itīs actual size, wich stealths does not. Therefor, because the B-1 doesnīt reflect as much as the truck, is not sure that it is detected at all. Of course that depends on how the pilot manuver and overall fly the plane. Is is also alot up to him if he will be detected or not. But for those radars that are able to scan very small objects, I somewhere read that the F-117 has a signature big as a seagull.
I donīt know if this answer fully satisfied you,but it gives a brief explaination how these things work.