fill, extend
礴 centers on spreading or filling a space completely, like water saturating an area. The given meanings "fill" and "extend" both point to this idea of pervasive coverage.
礴 is a phono-semantic compound: 石 (stone) suggests a solid, expansive quality, while 薄 likely contributes the sound ハク and carries a sense of spreading or extending. The exact historical development is uncertain.
Imagine a thin layer of stone dust (石) spreading out to fill every crack and crevice, like water extending across a surface. The right side 薄 suggests something thin and pervasive, so the whole character shows stone filling space.
For ハク, picture a huge, flat stone slab extending to fill a backyard (back -> ハク). The stone fills the space completely.