Don't know what you saw, but the Pres does not travel in smallish jets! It is usually a specially modified B747 Classic with the words "United States of America" on the side. It also sports the stars and stripes, of course has an N number and if the man is on board, the callsign is Air Force One.
I believe ther is also a B757-200 in same colors/livery which is usually used by other members of the cabinet and the media. However, I don't think the 757 is classified as a smallish jet.
Did you recognise the make and model? What color was it? Any markings? You sounded a bit vague