Norwegian company [Lufttransport](https://en.wikipedia.org/wiki/Lufttransport) have a scheduled route [BOO](https://en.wikipedia.org/wiki/Bod%C3%B8_Airport) – [VRY](https://en.wikipedia.org/wiki/Værøy_Heliport) using the [AgustaWestland AW139](https://en.wikipedia.org/wiki/AgustaWestland_AW139) **helicopter**.  This helicopter has 15 seats.  It's a bit heavier and longer than some small fixed-wing aircrafts, but certainly much narrower.  Dimensions from Wikipedia:

> Length: 16.66 m (54 ft 8 in)  
> Width: 2.26 m (10 ft 0 in)  
> Height: 4.98 m (16 ft 4 in)  
> Empty weight: 3,622 kg (7,985 lb)

For comparison, the [Twin Otter DHC-6 Series 100](https://en.wikipedia.org/wiki/De_Havilland_Canada_DHC-6_Twin_Otter#Specifications) has:

> Length: 15.77 m  
> Width: 19.8 m (wingspan)  
> Height: 5.9 m  
> Empty weight: 2,653 kg

The [Britten-Norman BN-2A Islander](https://en.wikipedia.org/wiki/Britten-Norman_BN-2_Islander#Specifications_(BN-2A_Islander)) is shorter, lower, and lighter yet, but as a fixed-wing aircraft, remains much wider than the AgustaWestland.  For most definitions of "smaller", the Britten-Norman would win, but if you're looking for the most compact "plane", the AgustaWestland helicopter flying to Værøy wins.