1 Terry: We're smaller than ants! How are we going to operate the shrink ray to reverse this?!
2 Steve: Don't worry, Terry. That's not a problem.
3 Terry: You mean the shrinking effect will wear off by itself after a while?
4 Steve: Nah. I mean there's no reverse mode to operate!

Well, think about it. What good would a reversal mode be on a shrink ray??

Typically, shrinking effects in fiction tend to have some sort of limited time span. It's a good way to create deadline tension when you're battling white blood cells inside someone's body, after all.