From a reader:
It is possible to party hard(y) without throwing your whole heart into it.
I’ve been to many a shin dig where I was partying hard, but it was not
something that I threw my whole self into the swing of things.
It is not possible however to party hearty without by definition partying
hard. If you are partying with your heart (and presumably soul) then you
can’t help but be partying hard (or has hard as you personally can muster).
Therefore, this is a matter of degree: to party hearty is to party harder
than to merely party hardy.