I'm not sure I can add much to this discussion but . . .
In the UK, most 'urban' caches are micro's (35mm film canisters usually), the bigger sizes are saved for rural locations. That's the point, what sort of terrorist would plant a bomb in the middle of the countryside ?
IMHO anyone who plants an urban cache bigger than a micro is a bit stupid . . . .
As far as paranoia is concerned I don't really want to go there . .. but, recently the BBC showed a very good three part documentary series 'The Power of Nightmares' - have a look at the BBC's description of it HERE