I wouldn't say that people are using too large a rear tire relative to OEM when is comes to, for example, a 215/40-17 front and a 265/40-17 or 265/35-18 rear.
OEM 16/17 is 215/45-16 and 245/40-17. Fine. But keep in mind that the rear rubber compound on OEM tires, whether they be Yoko or Bridgestone or Dunlop, it stickier than the front compound (this is my understanding).
Aftermarket tires are the same compound regardless of size. Thus, to "make up" for the lack of stickiness in the rear, you have to go with a wider tire, which explains using 215/40-17 front (approx. same tread width as the OEM 215/45-16), and a wider 265/40-17 or 265/35-18 in the rear. Further, a lower sidewall, which is true with the 265/35-18, will give the car less grip in the terms, assuming OEM suspension, because there is less flex in the sidewall, and thus it makes sense to use a wider tire to "make up" for this aspect as well.
ICBW, JMHO,
Andie