This point isn't necessarily true, depending on the goal. If the goal is to finish as highly ranked (in NW) at the end of the reset, it is a smarter play to take land from people you are already ahead of, because doing so costs less.

How does this cost less? Simple. You might spend "more turns to take less land", but at less risk of losing said land. Using more turns in this case isn't actually a downside, all my excess turns go into exploring or cashing anyway, since income generates every turn (except for techers). Being retalled means you lose CS, you lose tech, you lose a portion of built land, and attacking someone larger than almost always requires more jets and oil than attacking multiple small countries, such that this loss negates most of the potential gains from the landtrade.