I am quite sure a 25mm cube would print fine. I think that it is, as the second reply states, getting too hot generally under the print head. What it needs is a "go home" for 5 seconds at the end of each layer particularly when the layer is only a 5mm circle.

It is on a Wanhao Duplicator 3 using ReplicatorG. I don't know what slicer that uses but I will investigate.

Thanks for the replies. I will investigate further and see how the cube prints.