Well but comparing weights of Fullsize to Space Saving to Space Saver isn't really fair.
Like saying BMW 7 series (full size car) is heavier than Mercedes - Benz C class (compact executive) - you don't have to be a scientist to know that.
Apart from that one it's all right. Thing is Unicomp's internals are ALMOST the same as old IBM Model M, there was some cost cutting over the years and the quality control in Unicomp sucks compared to IBM which leads to some moulding marks on plastic and some squeaks here and there but it's not a big deal. Overall sound and feel is ALMOST the same. Differences are caused mostly by wear and the fact that some Unicomp keyboards (black ones) use single piece keys, while most IBMs are using 2 piece keys. In fact I couldn't say any difference between Customizer 101 (2 piece keys) and NIB Model M SSK. I've got both around the same time.