If you shrink the image down to 64x16 in the Construct 2 animation editor, then you will save memory (and loose quality :( )
If you are loading it in, and NOT shrinking it in the animation editor, and shrinking it down in the events/layout editor, then it will still be using the original 512x128 image in memory. (Though the payoff is that it's high quality, so if you ever resize the sprite to be bigger again in the game, it will look smooth rather than pixelated)