Longteng 16-300 or Longteng 18-400, which telephoto end shrinks more seriously?

Not quite the same, but the difference is very small. On the one hand, the telephoto end of this lens has indeed shrunk a little. In fact, the wide-angle end is the same, not so wide. On the other hand, even if it does not shrink, the gap between 200mm and 300mm and 400mm in perspective itself is not very big. Especially these days, many of them are high-pixel bodies. Even if the focal length is a little short, it doesn't matter if you cut it at all.