The antonyms for the phrase "gotten bigger" could be "shrunk," "diminished," "reduced," or "decreased." These terms denote a decrease in size or growth, rather than an increase or expansion. A reduction in size could be due to factors such as weight loss, deflation, or trimming, among others. Conversely, an increase in size could indicate growth, inflation, or expansion. When we use antonyms for "gotten bigger," we contrast the underlying sense of growth or expansion and give the sense that things have decreased. These antonyms provide a way to express a lack of growth or expansion in a concise and effective manner.