The formats we encode for www.wankzvr.com, www.milfvr.com and www.tranzvr.com are all mp4 files, but we encode them in two different ways. Our 1600p and lower resolution files are all encoded AVC (h264), this ensures they will play on the most devices, including in internet browsers. Our 1920p and higher (note: "4K" is 2160p) formats are encoded HEVC (h265). This encoding is more efficient, so we can have higher visual quality while having smaller files. Some devices and apps can't play HEVC files though, which is why we don't encode all of our formats that way.
File sizes are determined by the bitrate we encode the files in. Because it's more efficient, an HEVC file is roughly the same visual quality as an AVC file with double the bitrate. This drastic jump down in filesize is why our 1920p files are smaller than our 1600p ones. If they had the same encoding, then the 1920p files would be double their current size, and would be larger than the 1600p.