What kind is it?

I have sheared my 'Crimson Pygmies' because they aren't pygmy at all. They don't seem to suffer any ill effects. I chop them at random with hedge shears because I hate shrubs pruned into little balls unless in a formal setting.

'Rose Glow' needs to have main stems cut out at ground level or they lose their nice fountain shape. Took me three years to get mine looking normal again after DH whacked them off level....

If your Berberis thunbergii 'Atropurpurea Nana' aren't pygmies at all you probably got the atropurpurea without the 'Nana'.

bboy:

It's a plantius by former ownerus. Looks like a red leaf japanese barberry (B. thunbergii 'atropurpurea' most likely).

Those get much taller.

I think they just have grown bigger than the tag said they would. They were from a reputable garden center and they aren't nearly as big leaved or as big as my father's regular barberries but bigger than I expected. They are 4' wide by 3' tall and that is with being occasionally sheared.

My Spirea 'Magic Carpet' are also a lot bigger than I expected but aren't 'Goldflame' I removed 'Goldflame' to plant them and these are smaller and more delicate foliage wise but just bigger than expected.

No shrubs are fixed to specific sizes that are never exceeded, wider than high would certainly fit 'Atropurpurea Nana'. A mounding cloud-like habit versus the vase-like arching habit normal for the species would be the best indication.

Yes, that is how they grow and I shear them to keep the irregular cloud like shape rather than into little muffins or squares.