images not displaying full size

I checked the inspector, there’s only 1 set of images showing – here’s how the image URL looks

https://i2.wp.com/www.pix.dxtr.co.uk/…size=840%2C424

clearly showing some resizing, but what’s driving it? And more importantly, how do I stop it?

I’m viewing on a 24-inch (1920 x 1200) iMac by the way.