In addition to the confusion about x86 and x86_64/amd64, you should also know that -O[s123] cannot possibly influence the padding of structures, or it would mean you could not link a library compiled with -Os with a program compiled with -O3 (or viceversa).
In theory the compiler could change the padding of structures if it can prove that this never leaks out of the translation unit. Of course, printing the size or bits of the structure would inhibit that.
