Bend your knees....You don't need a whole lot of force as long as you have Ukes balance.
I agree with this comment. Once your weight is over uke, unless uke is shifting his weight backward (not proper ukemi for ikkyo) you should be able to simply bend your knees and bring him down.
If you are having a problem with this still, because you don't quite have uke when you are at the rest position (standing with uke controlled in ikkyo in front of you) then you can step at an angle to uke's body; through the armpit of the arm you are applying ikkyo to. This will bring most down, if they are taking the proper ukemi for ikkyo.