Roughly And In A Nutshell

Let's talk cased telescoped ammunition!

Top to bottom...

Textron's 5.56 LSAT cased telescopic round.
Standard 5.56x45mm NATO.
5.56x45mm case with the bullet shoved back flush with the mouth.
5.56x45mm case with the bullet shoved back and walls extended to the mouth.
5.55x45mm case with the walls extended to cartridge overall length.

Any dimensional errors are mine!  There's a few dimensions that are assumed on all the drawings I've found.

The standard round has 0.1044 cu/in of volume with the bullet in there.
Telescoped it drops to 0.0648 cu/in.
Extending the walls with the same case length doesn't change that.
Extending the case around the bullet gives 0.1069 cu/in, but the round ends up bulkier!

It doesn't seem like you're gaining much, if anything by shortening the round, huh?

Actual cased telescoped rounds, like the 5.56 LSAT, are not based on the 5.56x45mm case, it's fatter at the base and doesn't have much or any visible taper.  The LSAT round is a "push-through" extraction design with the following round shoving it forward out of a rotating chamber.  Textron has a video.

I eyeballed the dimensions of the 5.56 LSAT round from a Textron photo.  My rough drawing gives a volume of 0.0794 cu/in in a much smaller overall package than 5.56x45mm, but still 24% less a normal case.  Moving the bullet closer to the case mouth increases volume to 0.0814 cu/in, still 21% less than conventional.  Bet they're using a different powder!

Edited to fix some assumptions.