Do inner labia sometimes get bigger?

Yes: The labia can become larger over time due to many causes. It is called labial hypertrophy and if bothersome, can be treated with minor surgery, called labiaplasty.

Quite often: Yes they can get larger with hormonal alterations associated with puberty or pregnancy as well as due to physical stretching, varicocoeles, cysts, trauma, delivery, etc. Once enlarged and particularly in groomed individuals the exposed labia , may become irritated from mechanical contact and undergo metaplasia with thickening and darkening of the skin.

They can.: The labiae are elastic but can stretch over time. The tendency for this is probably genetic.