This is a very interesting case. I still think it's because of the high count of pixels being it's a bigger camera, but now I also believe other factors contribute to it as well after reading the other comments. I follow SoldierKnowsBest, who is a tech blogger on YT. He has been using the latest Nokia phone that has the 41mp count. I notice his pictures also look a little jagged, but I notice mainly or sometimes only when there are really bright colors and you're able to tell the detail. If it's an image where colors blend and don't pop you don't notice it as much if at all. He took a picture of a flower up close and the sky was in the background. The outsides of the flower were a little jagged. The picture wasn't bad overall.
Like the image above with the young man holding the phone. It looks fine until you see the phone he's holding and against his arm. Two different colors contrasting. But the darker colors like his shirt and the rest of the image just blend making it harder to see the outlines of things. Makes the image look nice and smooth overall or what we're used to seeing when uploading to Instagram. So it may be the combination of a Samsung S4 thing and high megapixel count.
Nexus 5 White