First to the weenies who think anything larger than a 22ga is torture - you know those needles the Red Cross and other blood donation organizations use to draw out a unit of blood from donors?  They're 15ga.  THAT is a large bore needle, as are 14's and 16's.  An 18 is not.  There are commercially available 10 and 12ga IV catheters available.  
Now, from my anesthesia/surgery standpoint.  If you have a trauma patient in the field, I will sing praises to your name if the patient comes to the OR with a 14-16ga IV in place.  Conversely, I will probably curse you endlessly if they roll in with a 22 in the ACF.  You may see the patient before they get into shock and when you can actually still find a vein.  If it's a trauma patient, and you see a big vein, PLEASE place something larger than a 20.  The further down the shock road they go, the harder it is to get a peripheral IV and the higher the chances that we're going to have to get central access.  I MIGHT not have to put in a central line if I have a peripheral IV that runs well.  Central lines are not an innocuous thing to do and have lots of nasty complications.
Those of you who think a 14-16 is "out of style" simply don't know what you're talking about, because in the proper situation, a really big honking IV (as we call them in the South) is a blessing.  A 20 on a trauma patient is simply too small for any significant volume resuscitation or for blood.  An 18 is better, but a 14-16 is da bomb.  Hey, if you don't want to flood the patient with fluids, fine - just turn down your flowrate.  But for those of us that deal with the patient shortly after you bring them in, my fluids/blood/FFP/colloids/multiple drips will go in much better with a larger IV rather than a smaller.
Oh - and as far as injecting IV contrast dye - using a larger bore IV in a larger vein such as the antecubital is desirable for several reasons, the main one being less chance of infiltration/extravasation.  IV contrast is thicker than IV fluid and harder to inject.  The tendency is to push it in - and of course when it's hard to push it in, most people just push harder, which leads to extravasation/infiltration, which is not a good thing with IV contrast.  Having a larger catheter in a larger free-flowing vein makes for happy patients and happy radiology techs.  But if all they have is a 22 in the hand, it'll do.  The tech will just complain more.