I think you are missing the point here. The unique feature of 8481 is the 14" screen on a small 13.3" footprint.

Comparison:

U31:
- Dimension: 32.8 x 23.3 x 2.54 ~2.85 cm (WxDxH)
- Weight: 1.78 kg (6 cell)
- Screen size: 13.3"

Acer 8481: 1.78 kg
- Dimension: 32.8 x 23.9 x 1.8 ~ 2.23
- Weight: 1.5kg (4 cell)
- Screen size: 14"

As you can see, 8481 is about the same dimension, except that it is thinner, lighter and housing 14" screen.

Also, please don't buy any notebook with Sandy Bridge that pair with a low end GPU such as GT520 or HD6470. The HD3000 in the SB is already as powerful as those GPU but running at lower power and less heat. If you plan to play game, you should not chose those low end GPU. But if you are not playing game, HD3000 is already sufficient.