His technique is no doubt very well adapted to his physique. To judge from the video footage of this race and the 100m race that is also to be found on youtube, he is of very slight build, at least as far as the upper body is concerned. Nevertheless he has the classic V-shape of the mesomorph rather than the stringy shape of the ectomorph.
No doubt his muscles are well enough developed to move his body through the water. I would guess that his leg muscles are particularly strong. His body position seems impeccable.
The World Championships in Rome should be very interesting. I wonder will Phelps be trying the 100m back in the US Trials? Stranger things have hapened.