Each point of minerals weighs 2 tons, I don't really know why, so a standard 5000 ton hold can hold 2,500 minerals.
Also the wiki needs love as it says "Most planetary installations, such as factories, mines, terraforming installations etc., weigh 25,000 tons but can be disassembled into parts of 5,000 tons each and transported separately"
I was under the impression that terraformers and research centers weighed significantly more.