O find
do AIX não parece suportar + 1M.
Na página do manual,
-size n
Evaluates to the value True if the file is the specified n of blocks long (512 bytes per block). The file size is rounded up to the nearest block for comparison.
-size nc
Evaluates to the value True if the file is exactly the specified n of bytes long. Adding c to the end of the n variable indicates that the size of the file is measured in individual bytes not blocks.
Você deve ser capaz de usar
find [dir] -type f -size +1048575c -exec gzip {} \;