Extricate has a point.
Newton's second law (F=m.a) is not only about Inertia. It is also about Force.
Bigger airplanes have more mass, that is obvious. But they have more Drag, too, right?
Same with energy (E=F.d). Mass has an effect in the distance, but so does force.
Therefore, if it takes more distance to slow down a big airplane than a small one, it means that the mass increase is greater than the drag increase when airplane size increases. Somehow, increase size is more efficient aerodynamically...
As for the secons question: increasing speed has two effects. One is that you exchange altitude energy for speed energy. However you will have to get rid of the extra speed energy before landing, which in turn takes distance... But exchanging both energies is not free. The air takes a "price" in the form of friction and heating an other forms enrgy loss, so you dissipare some energy when you dive, and also when you level off. You will need less miles to descend if you dive and increase speed 100 kt and later level off to decrease them again that if you just keep constant speed.
The second effect is the change in lift to drag ratio, which is what determines the descent angle. In the normal speed regime (speed above min drag speed) an increase in speed decreases this ratio, which increases the glide angle. For instance, at low speed you are more efficient and glide at 270 fpnm. Then at high speed you make 300 fpnm. And at vey high speed, 330 fpnm.