With any timber building, being a natural product, it will expand and contract depending on the moisture levels and different times of year.
The main thing with condensation is to increase the airflow this can be provided with ventilation. Static air will cause condensation, try adding air vents as high up as you can on either side of the cabin.
The cabin needs good ventilation, so if you can, open the door and the windows as often as you can to provide a good airflow, and this should help reduce the condensation.
It is also a good idea to treat inside the cabin, we offer a good treatment that can be used for this. This is the Impregnation Fluid, this will protect the cabin against damp and rot: