scale from small size screen to large one

I've a problem to scale the screen of a game from small size screen to a bigger one.

for expample I'm making a game with a 640,480 layout size but I want it automatically scales in order to work good with a 800,600 screen size..

If I use "Fullscreen in Browser" scale, It works good only from a 800,600 to a 640,480.. not viceversa.

How can I do?