The SmallInteger limitation is probably because as an optimisation they stored them as immediate values rather than generic objects, and Squeak shares this limitation (the newer Spur format extends this to one or more additional types depending on image bitness).
Related to references and the above, Eliot Miranda's blog may be informative as it provides both details and sometimes code for many of the changes that have happened in the Squeak (and now OpenSmalltalk) VM, e.g. http://www.mirandabanda.org/cogblog/2013/09/13/lazy-become-a...
Related to references and the above, Eliot Miranda's blog may be informative as it provides both details and sometimes code for many of the changes that have happened in the Squeak (and now OpenSmalltalk) VM, e.g. http://www.mirandabanda.org/cogblog/2013/09/13/lazy-become-a...