Basically mountains mark the region with high tectonic activity i.e tectonic plate's boundary. These mountains are formed through a series of underthrusting, uplift, folding and etc and this effects are further aggravated by plastic nature of the molten magma underneath the crust.
When mountains are elevated, the lithospheric slabs on either side of the mountain will be heavier than the elevated portion of the mountain due to foldings(folding causes the region to be thicker and therefore heavier).
The lithostatic pressure on these two region are higher then the lithostatic pressure imposed by the mountain. This will cause an inward movement of magma towards the base of the mountain further elevating the mountain that it already is. The reason why the magma moves in such away could be seen in a hydraulic system.
If the Earth's crust is to be denser, the lithostatic pressure on the asthenosphere will be higher and therefore more magma will distribute itself underneath the base of the mountain resulting in more buoyancy, therefore higher elevation is produced.
