There are a lot of good points, on both sides, here, perhaps a compromise would be in order, I too think that HP scaling in it's current state is detrimental, there's almost no scenario I can imagine that I'd rather go with a group of 18 than a group of 6 the way things are now, which, for a game with a storied history of (at least debatably good) alliance content, this is a bad thing, I think.

In large part it's due to the diminishing returns, extra TP feed without equivalent damage increase (skillchains being important in today's meta) or magic damage resistance, each extra person yields less and less benefit to the group, reguardless of how well geared they are, (though obviously if they are weaker, it makes it even harder on everyone), and also because trust grant a solid benefit without contributing to the HP scaling of the mob.

Perhaps the answer is that to answer this diminishing returns of bonus players, the hp scaling could mirror that, instead of approximately 100k HP per extra person (or whatever it is), each extra person could provide the monster consecutively less bonus HP, so instead of +100k HP per person, +100k, +80k, +60k, +40k, +20k, +15k, +10k, +10k, +10k, etc, etc, etc, and I'm not proposing these specific numbers, simply saying what if the bonus HP scaling was lower for each extra person, much like the benefit to a party is.