Complex sentence: For small or tall domes with less horizontal thrust, the thickness of the supporting arches or walls can be enough to resist deformation, which is why drums tend to be much thicker than the domes they support.
Split into simpler sentences: For small or tall domes with less horizontal thrust, the thickness of the supporting arches or walls can be enough to resist deformation. This is why drums tend to be much thicker than the domes they support.