Usage of memory by arrays

[unDEFER]

OK, without reallocation:

====================8<====================
void main()
{
     float[3] f;
     float[3][] x;
     writefln("float = %s bytes", float.sizeof);
     writefln("float[3] = %s bytes", f.sizeof);

     int before = MemoryUsage();

     int total = 100;
     x = new float[3][total*1000];

     int after = MemoryUsage();
     writefln("%dK * float[3] = %d Kbytes", total, (after-before));
}
====================>8====================

$ ./memory
float = 4 bytes
float[3] = 12 bytes
100K * float[3] = 2300 Kbytes

Why this so?