Memory layout of union of different sized member?

It lies on the first 4 bytes. From the C99 standard §6.7.2.1/14 (§6.7.2.1/16 in C11 and C18):

The size of a union is sufficient to contain the largest of its members. The value of at most one of the members can be stored in a union object at any time. A pointer to a union object, suitably converted, points to each of its members (or if a member is a bit-field, then to the unit in which it resides), and vice versa.

This implies that the address of all members of a union is the same.