Consider the following graph,

The plot has a steep slope from Li to Be but has much less steeper slope from O to F. This is due to the fact that

As we move right in a period the number of electron in outer shell increases with increase in atomic number. This result in inter electronic repulsions in the outermost shell and which in turn check the outermost shell from constracting too much. Therefore decrease in radius from O to F is not as much as that is in case of from Li to Be.