It's not just the different "under the hood size" that's a problem in this situation, but alignment-padding bytes added by the compiler inbetween struct members will have random junk data in them, the compiler will not zero-initialize padding bytes, unless you explicitely memset() the struct (and even then I wouldn't count on that the padding bytes aren't "tainted" later).

E.g. if you initialize a C struct or C++ object the "usual way":

Obj obj = { };

There will most definitely be junk in the padding bytes.