Waiting for a heatbed to get up to 85˚C for a relatively small part got me wondering why beds aren't hardware/G-code configurable for what area is heated? I'm sure it would be an increase in parts costs and electronics, but it seems that being able to just heat an area a little larger than the part(s) being built would save in time and energy use.
I've wondered that myself a while ago and fact is that such beds or silicone heating pads do exist. Usually these are quite large (and expensive) and usually referred to as "dual zone heat beds/pads".
As far as energy consumption; less area to heat is faster heat up times (depending on the control) and less energy consumed. For small prints this may be beneficial. The price of such beds are very high, so to break even you would have to print a lot. An alternative to buying would be to etch your own bed.