interesting buy orders

I was noticing that IRN had dropped today and realising that this is a no-brainer long term put in an order of 100000 units for 87 cents. I noticed that as the order came up on screen there was immediately below my order there was an order for approximately 70000 units. I did a little experiment and modified my order to 10000 units for the same price and the order below me changed to 26120 units. Does anyone have any idea what is going on? Obviously my orders triggered the order below me and the quantity of the order was dependent on the quantity of my order. But why and how?