Cars do not have large upright rear windows anymore because a sloping "fastback" design with a higher rear deck is more aerodynamic and gets an extra fraction of an MPG on the fuel economy rating.
I’m not so sure this is entirely due to fuel. A Volkswagen Golf with great fuel economy still has a big rear window and an FJ Cruiser has a tiny one and chugs gasoline. Many of the cars don’t actually have dramatic tapering roof lines, what they have is rising belt lines which cut into the window space.
Not all cars have such high beltlines that make you feel like you are driving a sherman tank. American cars seem to have these issues more than imports. Maybe its a way to shirk around some crash testing metrics by deferring to less visibility than to offer more visibility and potentially spend more engineering something just as strong?
My theory with the XC60 refresh with its huge, high, flat-top, snub-nosed bonnet compared to the old sloping one is that all that extra volume is for the fancy double/hybrid engines and so on.
Or maybe people just like to feel like their car is a rhino, but the same refresh also slightly lowered the roofline and the driving position feels lower too.
The other down side is that my wife always shouts that I'm going to crash when parking nose-in because the bonnet feels twice as long as it really is.
Larger windows mean more cooling and heating needed, and high window sills especially on sedans are needed because of how insanely high SUVs and trucks are these days, because people need to overcompensate.
Cars do not have large upright rear windows anymore because a sloping "fastback" design with a higher rear deck is more aerodynamic and gets an extra fraction of an MPG on the fuel economy rating.