OpenGL PBO texture uploading oddities - c++

Question

I'm basing my tests on this popular PBO example (see pboUnpack.zip from http://www.songho.ca/opengl/gl_pbo.html). Tests are done on PBO Mode 1 per the example.
Running the original sample, I found that on my NVIDIA 560GTX PCIe x16 (driver v334.89 Win7 PRO x64 Core i5 Ivy Bridge 3.6GHz), glMapBufferARB() blocks for 15ms even when the glBufferDataARB() preceding was meant to prevent it from blocking (i.e. discard PBO).
I then changed the image size from the original 1024*1024 to 400*400, thinking surely it would reduce the blocking time. To my surprise, it remained at 15ms! CPU utilization remained high.
Experimenting further, I increased the image size to 4000*4000 and yet again I was surprised - glBufferDataARB reduced from 15ms to 0.1ms and CPU utilization reduced tremendously at the same time.
I am at a lost to explain what is going on here and I am hoping someone familiar with such issue could shed some light.
Code of interest:
// bind PBO to update pixel values
glBindBufferARB(GL_PIXEL_UNPACK_BUFFER_ARB, pboIds[nextIndex]);
// map the buffer object into client's memory
// Note that glMapBufferARB() causes sync issue.
// If GPU is working with this buffer, glMapBufferARB() will wait(stall)
// for GPU to finish its job. To avoid waiting (stall), you can call
// first glBufferDataARB() with NULL pointer before glMapBufferARB().
// If you do that, the previous data in PBO will be discarded and
// glMapBufferARB() returns a new allocated pointer immediately
// even if GPU is still working with the previous data.
glBufferDataARB(GL_PIXEL_UNPACK_BUFFER_ARB, DATA_SIZE, 0, GL_STREAM_DRAW_ARB);
GLubyte* ptr = (GLubyte*)glMapBufferARB(GL_PIXEL_UNPACK_BUFFER_ARB, GL_WRITE_ONLY_ARB);
if(ptr)
{
// update data directly on the mapped buffer
updatePixels(ptr, DATA_SIZE);
glUnmapBufferARB(GL_PIXEL_UNPACK_BUFFER_ARB); // release pointer to mapping buffer
}