Fire is made in AfterFX, so we can say its vector. Water is made off vector shapes and bunch of layers with vector masks (50% vector). Air is made in AfterFX plus some clouds photos. Earth and metal buttons were made same way as water.
So only fire can be stretched any size without loosing a quality. Other icons can be stretched too (shapes are vector so they will have sharp edges), but some textures will be blurred.