There's two factors - rivets, and backplate thickness and material. The really old ones are made from that same heavy-duty brass-coloured tempered steel as the Model F backplates are, newer ones are not that rigid (I was able to bend a Unicomp one with my hands, and my upper body strength is... lacking).
Not entirely related, but my 1985 Model M is almost painfully stiff to type on, and I've heard others say the same of their early Ms, although it's probably a case of YMMV.