How to ask for feedback without sounding big headed???

[EdwardBear]

I applied for a job today. Its the only one I've seen in a year in my area that fits my experience perfectly!
I got an email an hour later saying 'Thank you very much for emailing your CV, unfortunately you have not been selected for interview on this occasion'
I think I have a good CV. I applied through a recruitment website where you fill in a form and attach the CV so no opportunity for a cover letter.
I'm so bewildered by the fact that I didnt even get an interview that I'd like to ask them for feedback.
How do I ask for feedback though without it sounding like I am saying 'I am gobsmacked you didnt give me an interview as I think I am great, why on earth didnt you?'
Any tips?

Just say its to help you with future applications, if they wouldn't mind letting you know why.

It could well be they'd already had enough applicants and didn't want to see any more.

Could you say something like 'to help me with future applications, could you highlight the areas in which I didn't demonstrate that I fulfilled the person specification [or whatever] for the job'?

I don't think asking for feedback would sound big-headed, unless you actually phrased it like that

What MadBad said, or similar. You could say that you felt you put in a good application so you would really appreciate some feedback to let you know where you didn't demonstrate that you fulfilled their requirements to help you with applications in the future.

But it may just be that they had already been overwhelmed with suitable candidates and decided to draw the line.