Yep - I've seen this a number of time - it's seriously cool!
It is an optical illusion.
1) you said the aircraft seemed 'smaller' - very probably it was further away than you thought - that helps, and the air could of been particularly
clear, that goes with
2) the aircraft is in a strong head wind - smaller light aircraft can almost have zero ground speed but enough air speed to fly - a jet liner moves a
lot faster, but a head wind will affect it.
3) Ground referance (or other referance) if you have nothing close to the aircraft to compare it with it can seem stationary - same deal with people
who insist the moon is 30% bigger when it's on the horizon - it's cos you can compare it to buildings and trees etc.
4) The best one is your own movement - you said you were in a car - that really helps - I saw 2 A-10s 'hang' when on a moterway - sweet!
So a perfect combo of those factors usually does it...
Of course others on this site will tell you it was a disguised UFO