.sizeof dynamically allocated array

[Adel Mamin via Digitalmars-d-learn]

import std.stdio;

void main()
{
     ubyte[] a1 = new ubyte[65];
     ubyte[65] a2;

     writeln("a1.sizeof = ", a1.sizeof); // prints 16
     writeln("a2.sizeof = ", a2.sizeof); // prints 65
}

Why a1.sizeof is 16?