When breathing is normal the breath airflow is 100% to 70%. Hypopnea is a decrease in breathing when hypopnea-airflow is 69% to 26% of a normal breath. There is the hypopnea index (HI) that can be calculated by dividing the number of hypopneas by the number of hours of sleep. But as far as hypopnea is closely related to apnea (total cessation of breathing) the apnea-hypopnea index (AHI) is commonly used to describe breathing disorders during sleep. AHI is an index of severity that combines apneas and hypopneas. AHI is calculated by dividing the number of apneas and hypopneas by the number of hours of sleep correspondingly.
When the apnea-hypopnea index is positive we generally speak of sleep-disordered breathing (SDB). SBD is a prevalent condition in the US general population that commonly associated with overweight and obesity. Most often problems with weight are caused by metabolism infringements and some eating disorders such as binge eating disorder.
Sleep-disordered breathing and all related diseases (sleep apnea, hypopnea, obesity hyperventilation syndrome, etc.) usually improve when the eating disorders causing overweight and obesity are properly treated. So there is a straight relation between weight loss and improvement in sleep-disordered breathing.
The figure shows the relationship between weight change and the apnea-hypopnea index. The data demonstrates that changes in AHI are related in a dose-response fashion to changes in body weight. A 10% weight loss is associated with a 26% decrease in the AHI, whereas a 10% weight gain is associated with a 32% increase in the AHI. So there are improvements in sleep-disordered breathing (sleep apnea and hypopnea) in case of weight loss and worsening of the disease in case of weight gain.
You can read more about the subject in Longitudinal study of moderate weight change and sleep-disordered breathing by Peppard PE and others.