This is a classic NTR plotline I’ve seen in a lot of doujins, but I don’t think I’ve ever seen anyone turn it into a game. It has really good NTR potential with a setup like this. Of course, the "size" difference is one of the most common themes in NTR.
There’s also a specific plot I’ve seen where, for example, when guys come of age, they get "measured," and depending on their size, they’re either allowed to reproduce with their girlfriend or not. If they don’t meet the requirement, the government assigns someone with a bigger "size" to take their place. It could be spun as something like “increasing the likelihood of having children due to a declining birthrate” or “better genetics."
There are a lot of variations of this plot in doujins. The boyfriend gets measured, and it results in NTR. That's the bottom line. But yeah, I don’t think I’ve seen it done in a game yet. Maybe it has and I just don’t remember. It definitely has potential. In my opinion, it’s at its best when the relationship is pure, like childhood friends, and then gets forced into NTR.
Good luck, Dev! I’ll be keeping an eye on this one.