¤ a guy was telling me that his samurai did beter in the mud with skinnier tires than wider tires why could this be? ¤

Samis are pretty light so it wont need as wide of a tire.  When you put a wide tire on, it has too little ground pressure.  With a skinny tire theres more pressure being put down in once place... better traction. 

I'm not sure this is the best for sand though.

Amilla

¤ what about a jeep or truck ¤

it seems bass ackwards to me.
skinny tires would act like a pizza cutter? wouldn't you just get better traction if it cut down to something hard?
i always thought wider would be better to spread load and gain traction, like tank tread.
i've heard this same discussion regarding snow and ice.
but then i'm not a mud whomper and don't do a lot of snow stuff either.

i run tsl pizza cutters (9.5 wide) on my tracker and love them!  they air down great for rocky terrain and flex nicely without dropping the pressure too low.  i really don't believe that having a wider tire would held such a small light truck in the mud all that much, this whole wide tire floating in the mud theory doesn't sound completely wrong but realistically there's not much power behind these things then once the tires are a foot into the mud it's just more tire to push through and more weight to turn.