It’s been a long time since I’ve seen it but the movie Mimic had bugs that had grown to the size of a human and taken on a vaguely human form in order to hunt us.

The movie used the reasoning that the bugs had developed basic lungs which enabled them to grow past the limits of their usual breathing apparatus.

No point to make here, I just remember it being cool that they put a small amount of thought into why the bugs could grow to human proportions.