Suffixes called augmentatives are added to words to indicate augmentation. One example in English is:

ball --> balloon

The Spanish suffixes on (masculine), and ona (feminine) denote augmentation:

libro - book ---> librón - large book
hombre - man ---> hombrón - big, strong man
grande - big ---> grandón - really, really big

Note: Feminine nouns generally take the suffix on and become masculine:
una mujer - woman ---> un mujerón: a big, strong woman

Many people are not aware of this rule, so you will often hear sentences like the one below:
La enfermera es una mujerona.
The nurse is a big, strong woman.

Other augmentative terminations are: -azo, -acho, -onazo, -achón, -ote, -astro, -aco,

Augmentative terminations have no determinate meaning: they are vague and indefinite and consequently when preciseness is required we must use the adjectives "grande," "pequeño," etc. (which can be employed conjointly with the terminations), as:

Me dió dos librones gruesos y tres libritos delgados.
He gave me two heavy big books and three small ones.