Don't forget everyone...according to the lore the primals power level matches what energy they have been fed (crystals, etc).
Also, if I remember correctly, the primals are just a manifestation of energy into physical form- when they die they go back to the aether and can come back. It is entirely possible bahamut breaks out of the coil at the end, but with very little energy compared to the opening video...thus he can only maintain the small size. The size is (as i understand it) incredibly variable.

Also do not forget that we have already seen primals manifest body doubles (ifrit and to a lesser extent garuda)