Ah, yes, my sister had this same issue with her little pygmy (Rue). The door is ridiculously tiny in comparison to her stout little body and we couldn't believe she was going through it to get inside until the day we actually saw her do it - how she didn't get stuck I will never know. For her the fix was beyond basic -- there was an old wood splitting wedge that had been used as a door prop in another building and when placed inside the run with the thin end of the wedge pointed up and out the opening so it poked out about an inch or two (the wide end of the wedge resting on the ground inside the run) was all it took to convince Rue she would no longer fit (how or why IDK). The chickens have no issue hopping up/around the wedge to go through the door as they always have, but Rue no longer tries it (she does, though, watch for the slightest chance to get through the people door when chores are being done if someone doesn't know about her habit or forgets what a sneaky little buggar she is.