You're just wrong. You're looking at the wrong numbers. The perplexity score of a model with twice the parameters in half the bits (4bit) is FAR LOWER (ie better).

If you are limited to X RAM and have two 16bit models of size 4X and 2X then the 4X model in 4bit will always be far superior to the 2X model in 8bit, with far lower perplexity.

Compare 13B's 4bit perplexity of 5.3607 to 7B's 8bit perplexity of 5.9069. That is over 0.54 lower perplexity for the same RAM amount by using 4bit! That is MASSIVE!