An aerosol-generating system comprising: an aerosol-generating article having an upstream end and a downstream end , the aerosol-generating article defining a longitudinal direction between the upstream end and the downstream end, the aerosol-generating article comprising: an upstream segment disposed at the upstream end of the aerosol-generating article, the upstream segment comprising an aerosol-forming substrate , the aerosol-forming substrate comprising a substrate outer surface having a substrate outer diameter (D2); and a downstream segment disposed at the downstream end of the aerosol-generating article, the downstream segment having a downstream segment outer diameter (D1), the downstream segment outer diameter being greater than the substrate outer diameter; and an aerosol-generating device comprising: a device cavity comprising a device cavity inner surface having a device cavity inner diameter, the device cavity being configured to receive at least the aerosol-forming substrate of the aerosol-generating article; and at least one heating element configured to heat the aerosol-forming substrate when the aerosol-forming substrate is received in the device cavity, wherein, when the aerosol-forming substrate of the aerosol-generating article is received in the device cavity, an airflow passage is defined between the substrate outer surface and the device cavity inner surface, the airflow passage extending in the longitudinal direction along the length of the aerosol-forming substrate. an aerosol-generating article having an upstream end and a downstream end , the aerosol-generating article defining a longitudinal direction between the upstream end and the downstream end, the aerosol-generating article comprising: an upstream segment disposed at the upstream end of the aerosol-generating article, the upstream segment comprising an aerosol-forming substrate , the aerosol-forming substrate comprising a substrate outer surface having a substrate outer diameter (D2); and a downstream segment disposed at the downstream end of the aerosol-generating article, the downstream segment having a downstream segment outer diameter (D1), the downstream segment outer diameter being greater than the substrate outer diameter; and an upstream segment disposed at the upstream end of the aerosol-generating article, the upstream segment comprising an aerosol-forming substrate , the aerosol-forming substrate comprising a substrate outer surface having a substrate outer diameter (D2); and a downstream segment disposed at the downstream end of the aerosol-generating article, the downstream segment having a downstream segment outer diameter (D1), the downstream segment outer diameter being greater than the substrate outer diameter; and an aerosol-generating device comprising: a device cavity comprising a device cavity inner surface having a device cavity inner diameter, the device cavity being configured to receive at least the aerosol-forming substrate of the aerosol-generating article; and at least one heating element configured to heat the aerosol-forming substrate when the aerosol-forming substrate is received in the device cavity, a device cavity comprising a device cavity inner surface having a device cavity inner diameter, the device cavity being configured to receive at least the aerosol-forming substrate of the aerosol-generating article; and at least one heating element configured to heat the aerosol-forming substrate when the aerosol-forming substrate is received in the device cavity, wherein, when the aerosol-forming substrate of the aerosol-generating article is received in the device cavity, an airflow passage is defined between the substrate outer surface and the device cavity inner surface, the airflow passage extending in the longitudinal direction along the length of the aerosol-forming substrate. <EOS>