Just tried this on my machine and the problem seems to be that you need to extract the objects from the archives before adding them to the new archive:
ar x libsmall1.a
ar x libsmall2.a
ar rcs libbig.a *.o
Simply running ar rcs like you did produced an archive which contained two .a files, but tools (e.g. nm) were unwilling to look deeper into these files.
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