If making phone calls is all you really care about in a phone, why even bother with a smart phone at all? There are plenty of very compact phones on the market that are great for just making phone calls.
The reason to even own a smart phone is because of the extra functionality of the phone. Photos, web surfing, games, video, and what not all are done better on a nice smart phone with a decent size screen. The smaller screen makes those functional requirements of a smart phone harder to do. It just isn't done well on a small screen. Still, part of the reason of having a cell phone is portability. Although people were able to port around the original Motorola brick phone just fine back in the 80s. Even the biggest phablet is smaller than that monster. 4.5 to 6 inch screen is about the sweet spot for phones. Smaller screens than that just too small and bigger, while always nice for the other functions, just starts to become a tad bit too cumbersome to tote around by getting into that realm of motorola brick phone size almost.