They are often not stretched very much at all, there was just over an inch in length difference between my eZee
Torq 1 and the unpowered bike I had at that time, and the same is true of many of the Panasonic powered bikes.
The reason is such things as juggling of the frame tube angles, shortening of the stem to seatpost difference with
compensation at the stem forward extension.
The main reason why batteries were behind the seat stem originally was that they used to be far too big and
heavy for anywhere else, at first SLA, then still quite large and heavy NiCad and NiMh, then the earlier bulky
lithium ones. Long range 37 volt lithium batteries from 14 to 18 Ah are still too large for other locations, so all
these things have conspired to make that the favourite position still.
Rear carrier batteries have some disadvantages regarding weight distribution, panniers accommodation and
height etc, and 37 volt batteries over 10 Ah are difficult to fit into the frame triangle, leaving not many other
options.
.
