This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
Altitude is largely responsible for the dwarf habit of alpine plants. When these are found at lower altitudes, or in moist situations, they are taller. The plants that grow on rocks are in general not very tall, nor are the crevice plants in exposed situations, but in lowland districts in wooded areas Orpine may be luxuriant and grow to a height of 2 ft. The habit of the plants is intimately connected with their height; and mat, cushion, or rosette plants are consequently more or less dwarf. Thus Vernal Whitlow Grass is rarely more than 5 in. in height even in flower; Cheddar Pink, 8 in.; Field Mouse-ear and Sandwort Spurrey, both trailers, are 6 in. and 2 in. respectively; Bird's-foot is 2 in.; Horse-shoe Vetch, 6 in.; Rue-leaved Saxifrage 6 in.; Meadow Saxifrage and Navelwort, growing in more sheltered situations, reach a foot or more in height.
Dry conditions limit not only the height but also the extent of the branching or size of leaves.