One possible theme could be growth. The little brother might start out small and weak but grow in strength, character, or influence, becoming 'bigger' in different ways as the story progresses.
Another theme could be the underdog. Since he's the little brother, he might be underestimated, but then he overcomes odds and shows that he's more than what others thought. This theme of defying expectations and rising above is quite common in stories. Also, the theme of family loyalty can be there. He, as the little brother, might be doing things for the sake of his family, which in turn makes him 'bigger' in the eyes of his family members and in the overall story.
A third theme could be self - discovery. The little brother may be on a journey to find out his true potential, and as he discovers more about himself, he realizes that he can be 'bigger' in terms of his capabilities and contributions. For example, he might find out he has a talent for art or music that not only enriches his own life but also impacts those around him positively, making him a 'bigger' presence in the story.
Chapter 182, "Director Sun Still Knows How to Stir Things Up", is the clearest early answer because it turns Su Yinwan's situation into a concrete plot event. It opens with At this moment, Qi Tianyu, who had just arrived in the Capital,...
Chapter 384, "To celebrate my selection, live streaming tomorrow night.", is the clearest early answer because it turns Shen Lan's situation into a concrete plot event.
Chapter 135, "Lai Fu is Missing!", is the clearest early answer because it turns Aunt Ying's situation into a concrete plot event. Early on, The sun blazed down mercilessly, but thankfully, the path to the.... By the end, Are you healed from your injuries?" Zhao Lang smiled and nodded,....
Chapter 438, "The Upper Stream is Mine, The Lower Stream is Yours", is the clearest early answer because it turns Yours's situation into a concrete plot event. The chapter turns that into something concrete by moving from I'm sorry," Shang Shuyue squeezed out three words through clenched...
Chapter 202, "Jiang Li Official Announcement?!", is the clearest early answer because it turns Daniu's situation into a concrete plot event. The chapter gives readers a real answer by opening on Jiang Li couldn't believe it, "Just to take a picture?" "Yes, just to...
Chapter 229, "He's Actually Brother Liu, the Watermelon Seller?!", is the clearest early answer because it turns Huo Zhenhua's situation into a concrete plot event. The chapter gives readers a real answer by opening on Huo Ziyang's face had already turned deathly pale.
Chapter 66, "This Sister Really Can Hustle!", is the clearest early answer because it turns Director Sun's situation into a concrete plot event. It opens with I can't take it anymore, why did the young master even find a pair of...
Chapter 183, "Sacrifice Oneself, Break Them Up!", is the clearest early answer because it turns Princess's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Seeing that he held a blue ball identical to Su Yinwan's, Jiang Yan...
Chapter 108, "Without accumulating virtue through words, calamities of bloodshed will only increase, not decrease.", is the clearest early answer because it turns Was Jiang Li's situation into a concrete plot event.
Chapter 245, "More Than Meets Your Match!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Jiang Yan became even more smug upon hearing this.