New cars keep getting heavier and heavier! Just check out the weights on the new Camaro and Mustang. Brakes need to get bigger to compensate for this as well. ABS and traction control are a big help as well.
Your tire question I believe is: the overall diameter of tires for new Super Cars (Camaro, Corvette, Mustang, Challenger) are shorter than 1st & 2nd Gen. muscle cars?
If this is your question than take a look on Tire Rack at the OD for the tires these cars come stock with. They range from 26" to almost 28" in diameter. Many come stock with 20" wheels and rather than having a shorter sidewall they (the car manufacturer) have increased the sidewall diameter of the tires so not to loose ride equality.
If your referring to the tire and wheel width, than a 12" wide compared to the original 7.00X14 bias pizza cutter tires these came with 40+ yrs ago is a bit of a difference as we know.
The widest tire available with a 200 tread wear is a 345 which fits on a Viper with a 13.0" wide rear wheel.
I believe we'll see the wheel and tire sizes level off at 20". I really don't see 21" and 22" wheels and tires being stock on new cars in the future.
Lots of tire options in 18", 19" and 20" sizes now from a wide variety of tire manufacturers. The 19" and 20" tires are generally taller in OD than 18".
In general 18" tires are 25.0" to 26.0" in OD and 19" and 20" are 26.0" to 27" in overall diameter.
Not sure if this answers your question entirely, but hope it helps!