13.001, 13.01, 13.1
to put it more visually, i'll show in fractions.
13.001 = 13 1/1000
13.01= 13 1/100
13.1 = 13 1/10
a piece out of 10 would be bigger than a piece out of 1000 because in that case there are many more pieces and they'll have to be smaller to make up the whole.
hope this helps.