Responses
Actually, Endo's dialogue before and after the kiss implied that the reason he shifted was to set the mood for Hiriki's Wall Slam/ Disinfection Kiss cliche. And the only to play the cliche to it's fullest - i.e."one person shoves another against a wall and looms over them" - required Endo to be taller than Hiriki.
Further, Endo even stated when Hiriki started to ask about the form shift, "It uses too much power so I'm not gonna do it again", implying that Endo used his own power to shift rather an power he might have gained from Hiriki or any other viable source.
I think in one of the previous chapters Endo temporarily turned demon to kiss Hiriki once. Then again what's stopping Verde from dragging the bad guys ass back to the demon world and destroying the his portal.