Quote Originally Posted by Kraco
It rather seems to me it's because he's lacking that kind of control that he forms big objects out of it and flings them at his opponents. The man sized metal spikes were the smallest effective forms, I suppose. This would suggest his control over the movements is pretty limited and he can't give them more than a limited speed either. So, anything small would just tickle the opponent (or jam puppets). Still, I'm not saying his battle tactics with the iron sand were anywhere near optimal.
Sasori underestimated Sakura and Chiyo. He was hoping to finish off the fight quickly and save chakra so that he could join in the Naruto/Deidara fight. He says something along the lines of "I guess I'll need to use more chakra" before he uses that iron-tree attack.