Why is the size smaller than it should be?

wrong_size.d

import std.stdio;

struct Vector(T)
{
    T x, y, z;
}

struct Node(T)
{
    Vector!(T) position;
    T mass;
    Node!(T)* next;

    // if this is commented out the size is correct
    Cell!(T)* cell()
    {
        return (cast(Cell!(T)*) &this);
    }

    /* On 64-bit system:

       with cell() included: (Cell!(double).sizeof is only half of the sum of the element sizes)
       Sizes body = 72 cell = 56 node = 40
       Children Size = 64
       Cell must be at least 120
       Cell sum  120

       without cell() included: (Cell!(double).sizeof = sum of the element sizes)
       Sizes body = 72 cell = 120 node = 40
       Children Size = 64
       Cell must be at least 120
       Cell sum  120

     */
}

struct Body(T)
{
    align Node!(T) node;
    Vector!(T) velocity;
    bool ignore;
}

struct Cell(T)
{
    align Node!(T) node;
    Node!(T)* more;
    T critRadius2;

    union
    {
        Node!(T)* children[8];

        struct QuadMatrix
        {
            T xx, xy, xz;
            T yy, yz;
            T zz;
        };

        QuadMatrix quad;
    }
}

void main(string[] args)
{
    writefln("Sizes body = %d cell = %d node = %d", Body!(double).sizeof, Cell!(double).sizeof, Node!(double).sizeof);
    writefln("Children Size = %d ", Cell!(double).children.sizeof);
    writefln("Cell must be at least %d ",
             Node!(double).sizeof + (Node!(double)*).sizeof + double.sizeof + (Node!(double)*[8]).sizeof
        );

    writefln("Cell sum  %d",
               Cell!(double).node.sizeof
             + Cell!(double).more.sizeof
             + Cell!(double).critRadius2.sizeof
             + Cell!(double).children.sizeof
        );
}