Hmm my Single textured Landscape and the exact same size as yours (127x127, 2x2, 32x32, 8129x8129, 1024) uses 4GB aswell (with 2 Landscape its 8GB, while PIE it goes over 16GB!). In a clean new project the default Project memory usage is aprox 700mb, after adding a new 8k landscape without anything else it ramps up to about 2GB. I have been trying to recreate the conditions within the new Map (enabling Mesh Distance Fields, equal Lighting etc.) but the landscape memory usage keeps at about 1,3GB, while in the other Project every Landscape adds aprox 4GB.
So I agree, apparently there is something that is tripling the landscapes memory footprint.