I don't have the energy to write this fully right now, but at the very least I can talk about this in detail.
Firstly, Todd is very guarded about his fetishes around Falm. This is largely out of embarrassment of the fact that he literally finds the daily horror micros go through to be attractive. The very idea of sharing that in-depth with Falm makes him cringe and really feel guilty.
That being said, it eventually will be revealed. There will be obvious bafflement period, bits of mockery on behalf of the sarcastic rabbit. Then more or less just...trying to comprehend it and failing heavily. It is just not something that Falm can really wrap his mind around. He already has a younger sister who, even if she had only started shortly before he left, was incredibly sexually open and pretty much would do some very kinky things with people her age as part of experimentation. But that's just mostly normal sexual stuff.
The idea of being stepped on, eaten, treated like a sexual object by someone more massive than himself...he has a very normal reaction to that: terror. To be under the power of a greater being and used is something he has witnessed and lost friends to. So there is a level of discomfort with it. That said, despite being a sarcastic ass to his boyfriend about it, he doesn't...genuinely kinkshame Todd or look down on him for it? He just kinda accepts his boyfriend is not only a strange being that is basically an alien to him, but is also a weirdo...in a good way.
That said, if Falm did try to indulge Todd, it would be awkward. Because 'act like a giant' basically would amount to Falm trying to put himself in a mindset of 'I am staring at a tiny, powerless being and deciding to torture it.' Basically, imagine the most cringe sizetrash dialogue and combine it with someone who is awkwardly trying to improv a topic that is the very opposite of who they are. It would be like watching a train wreck carried out by someone unironically using the phrase 'you like that, little man?'
So basically, Falm and Todd would be dying inside but unable to really say anything because Falm wants to make Todd happy through indulging these incredibly terrifying and genuinely odd fantasies of his and Todd is too appreciative of his boyfriends efforts to say anything about how he makes the terrible size videos he used to occasionally watch on YouTube look like works of Shakepearian theatrical art by comparison.
If I had the energy, I would write it. And it would literally be the best worst thing I'd have ever created.