The padding and alignment of members of structures (§3.5.2.1)
Structure members are aligned to the minimum of their own size or the current packing size. For 16-bit targets, the default packing size is 2. The default corresponds to the /Zp2 command-line option. The default packing size is 4 for 32-bit targets. See topic , “Storage and Alignment of Structures,” for more information.