The size of a structure is greater than the sum of its parts because of what is called packing. A particular processor has a preferred data size that it works with. Most modern processors' preferred size if 32-bits (4 bytes).

This should save a lot of memory on many method calls.

An implicitly typed local variable is strongly typed just as if it had been declared the type, but the compiler determines the type. The following two declarations of $i are functionally equivalent:
int $i = 10; // Explicitly typed.
auto $i = 10; // Implicitly typed.