This manga genra is getting more popular, I am seeing more like it, maybe it's not so new. I remember from 10 years ago, we had K-ON and even a character from Negima. Now it's gone full onee-shota and even Eiken which was soft porn most of the times. Doujin authors could literally turn ever chapter of that into a new doujin.
I think authors need the MC to be this meek, shorty to show us how weak he is and to empathize how hot the Onee-san is. If it was a guy as tall as the Onee-san, then we would never have that "death by tits" moment. And this manga seems to really want to rely on that fanservice, just from the cover.