I would hazard a guess that it cost OCUK around the low £4.xx per box mark to send a box with DPD.
If there isn't a £4.xx margin in the item, you won't be getting it via DPD.
Saying that, I would want a lot more than £4.xx GP on an item before I offered free carriage to that amount so imagine it's not just based on the size of item, there will be price implications in the decision as well.
Actually its just based off weight and price bands (royal mail has price limit and weight limit), we make losses on forum shipping all the time, main problem with free shipping is people tend to buy odd bits and bobs instead of bundling it in to 1 parcel which costs even more.
Chances are the reason it went DPD is it was more expensive, or someone put the weight in the system too high (if its a light item)