If you take three different sized leds xp-e, xp-g, xm-l and put them in the same sized reflector, and give them the same amount of current. The smallest led will throw the furthest. basicly two reasons. The bigger ligthtsource you got, the bigger reflector you need to focus the beam into throw. Just the same way as a big man needs a big mirror in front of him to see his whole body. Yes he can take a step back, it helps the same way as a deep reflector does. And the bigger leds will have less surface brighness because they get less watts pr square mm than a small one. So throw is determined by the size difference between the lightsource and the optic/reflector, and the intensety of the lightsource.
Those old halogens has a small lightsource compared to leds. And high surface brighness, so it's easy to get some throw out of them.