Japan has a genre called "brocon" (brother complex), where a sister is excessively fond of her younger brother. The phrase fits that mold perfectly. She is both amazed ("maji de dekain") and eager to show him off ("mi ni kona"). This is often played for laughs when the brother is actually oblivious to his own size or the sister鈥檚 bragging.
In the vast ecosystem of Japanese internet slang, anime dialogue, and light novel titles, certain phrases transcend their literal meanings to become cultural touchstones. One such phrase that has piqued the curiosity of language learners and anime fans alike is (銇嗐仭銇紵銉炪偢銇с儑銈亜銇戙仼瑕嬨伀銇撱仾銇�). Uchi No Otouto Maji De Dekain Dakedo Mi Ni Kona