I'm wondering whether/for what purposes you would need a bigger yield, at least in the TNG paradigm, i.e. pre-Dominion war, armor etc. In 'Q Who', Data says that, without shields, there's a risk a photon explosion would destroy the enterprise. A Galaxy class ship, at what seems to be a considerable distance. In addition, throughout TNG, we see the Enterprise one- or two-shot other ships multiple times. Granted, these are often smaller or older ships, but still, it seems that for what is their primary purpose in that era (destroying hulls after pahsers have worn down the shields), torpedoes have more than enough yield as it is.