The checksum is computed on the basis of the content of the file, thus it can be different for the files having same size if the file content is different.
bash-2.05# echo "hello1" >/tmp/hello1
bash-2.05# echo "hello2" >/tmp/hello2
bash-2.05# cksum /tmp/hello*
1144619315 7 /tmp/hello1
1179677882 7 /tmp/hello2
Size of both the files is same (i.e. 7), but there checksum is different.