Understanding gc memory profile report

[John Burton via Digitalmars-d-learn]

I wrote this simple program to test out my understanding of 
memory allocation :-

     import std.stdio;

     void main()
     {
         int [] array = new int[250];

         writeln(array.length, " elements ", array);

         // Append one value to the array
         array ~= 123;
         writeln(array.length, " elements ", array);
     }

I compiled it with 'dmd test.d -profile=gc'

After running it, the profile report was :-

     bytes allocated, allocations, type, function, file:line
                2000               1 int[] D main test.d:5
                   4               1 int[] D main test.d:10

This is not how I expected it to be. I would have expected that 
the runtime either did not have to allocate at all at line 10 to 
add a new element because there was already space or it would 
have to allocate space for the new enlarged array and copy the 
array to it, in which case I'd expect it to allocate 2004 bytes 
(or more) to copy the enlarged array in to.

I would not expect that it could have allocated 4 bytes to add an 
item separably from the original 2000. Is there some way that the 
runtime can grown the original allocation by 4 bytes and that's 
what I'm seeing? If so, is there a limit to how much it can do 
this?

Can anyone help me understand what is going on here?