0.8 is the larger number.

If you restate the numbers in words it makes it easier to figure out.
0.008 is read 8 thousandths -- think a circle divided into 1000 pieces and you have 8 of them

0.8 is read 8 tenths -- think a circle divided into 10 pieces and you have 8 of them

Of course the circles must be the same size to start with.