I've had a whole bunch of flats from really small pieces of glass or wire in the Philadelphia area. These things went through my kevlar belted tire (bontrager race lite) AND 2 or 3 tire liners (I used slime brand and tuffy brand).

I switched to mountain bike tires and wheels, and I no longer get flats from these 3-4 millimeter long objects, because the depth of the tire tread and casing is about 5 millimeters. Problem solved! Tuffy liners and slime liners are made of hard/flexible plastic of some sort which in my experience doesn't do much to repel sharp metal or glass objects, although it does help increase the "tube to road" distance.

Big tires are just as vulnerable to nails as little tires are though. In 10 years of biking on a regular basis, I've gotten two flats from nails, one on a typical low end MTB tire and one on a kevlar belted road tire.