Hola Marcos

When we form a noun using the augmentative suffix -ón, the new word is always masculine, regardless of the gender of the originating noun. This is because nouns ending in -ón are generally masculine (except for -ión, normally feminine).

Saludos

Inma

Hola Chris

Unfortunately there is no rule for that - it is a bit random. For some words we make the augmentative in a different gender to the original word. 

In the case of soltero/soltera, though, it is clear that you need to make two different augmentatives, one for masculine -ón, and one for feminine -ona, because we need to distinguish between male and female. For película, as it is a single gender (inanimate) we just make one augmentative and if we choose to make that new word using -ón, then that new word becomes masculine. 

The augmentative suffixes are a bit random as we explain in the lesson. Some areas tend to use some and other areas prefer to use others as with the diminutives. With the lesson we want to illustrate and make students aware of different ways to make these new words and recognize them. 

Saludos

Inma