The equation for force of gravity is F = GMm / r² where

• G is the gravitational constant
• M is the mass of the planet
• m is the mass of the object being weighed
• r is the radius of the planet / distance between the centers of mass of two objects

The equation shows that gravity scales linearly with mass, and scales inversely with the square of distance. The article says K2-18 b is 8.6 times Earth's mass, and 2.6 times Earth's radius. So the weight of a 100 kg mass would be:

F = 100 × 8.6 / (2.6)²

which works out to a weight of about 127 kg, or 1.27 times heavier.