There’s big, then there’s bigger, and biggest. These words have names.

Bigger = comparative. It compares one thing to another. Smaller, is also a comparative and so is the word ‘less’. ‘More’ is also a comparative.
Examples of the Comparative
Taller, wider, narrower, fatter, heavier, thinner, cleverer, shorter, sweeter, uglier, healthier, unhealthier, wealthier, poorer, lovelier.
Not all comparatives can have -er on the end. Sometimes you just have to add ‘more’ or ‘less’ at the start.
more sour, more beautiful, more attractive, more unfortunate, more delightful, etc.

How do we write about it? In ‘Mother Any Distance’, Armitage uses the comparative to show that the ‘distance’ is ‘greater’, as if the gulf between him and his mother is widening – they’re growing farther apart.
Notice what two things the writer is comparing and comment on the context and relationship.

Anything that is ‘greatest‘ is superlative. This should be easy to remember as superman is the most super of all superheroes. 

How to write about it?
The use of the superlative shows that this is the ‘most’ important thing of all.