How would breathing through a straw simulate chronic lung disease caused by smoking?

Airflow resistance. Think of the lungs as a giant balloon. The critical point in the system is the neck of the balloon. The narrower the neck, the harder it is for air to flow in and out. In chronic obstructive lung disease the bronchial tubes are narrower due to mucous, swelling, inflammation and sometimes loss of support structures. The straw analogy relates to the airflow limitation through smaller tubes/airways.
Mimics it. When you smoke, your air passages become clogged with both temporary debris (inflammation, swelling, mucus) and permanent changes (scarring, narrowing). When you breathe through a straw, you are mimicking trying to get air through a small, narrow passage.