graphics2d.scale to make full screen

suppose we have a game with 320 width , 240 height, with xscale and yscale to 1 then I get the aspect ratio and and I calculate the new scale that match the whole screen keeping the aspect ratio, that works the game is scaled but if the new height and width is 1500 x 1500 im only seeing 320x 240 part on the screen,

any idea why this happen?, how can I scale the game to create some kind of full screen support?