Once again, reigniting the request for the two tonics mentioned in the title to work with mounts (and resize them too).
Back in the day, due to buggy interactions between the newly released mounts and already existing transformation tonics, the combination of the two was disabled. This made sense, since there was a lot of tonics that would have needed tweaks, and it was an overall hassle to deal with.
About a year ago, there was a time where a bug was discovered involving holosmith and necros, which would allow players to use mounts while the game still considered them unmounted. This, admittedly, came with its problems, like players being able to literally use some combat abilities while still on their mounts, but it also allowed for the usage of both the Miniature and Embiggening tonics, which made for hilariously over/undersized mounts. The bug was found and patched extremely quickly (because people, instead of just enjoying the miniature or giant mounts, decided to abuse the combat part of it), but during the time when it was reproducible, there was no instance where the mounts themselves looked strange or janky, merely because of the size change. The models looked perfectly fine. So why not just let people mount while under the effects of these two tonics specifically?
You could have argued that "it's just not feasible to allow only some tonics to work", and I would have agreed, up until now. The thing is that, not long ago, some COMPLETELY ad hoc tweaks were added to certain tonics; namely the fact that mounting while using them now, instead of prompting a message telling the player that they can't mount, it just transforms the player back and mount them.
This really does make me wonder: if completely arbitrary behaviors like this can be decided for different tonics, why disable all of them while mounting? It makes sense for some tonics (like the Kodan or watchknight ones), since they don't have animations for mounting, but what about the ones that either turn you into an already playable race, or straight up just alter your size?
The final argument that has been made in the past against this idea, is that "if it were added, wouldn't that make performance worse?". Put it simply: no, it shouldn't. At least not significatively. Not all players have these two tonics, and amongst the ones who do, I rarely ever see them outside of roleplaying hotspots. Even then, the size of a character has little to nothing to do with how lag inducing they are, since character size doesn't affect particles at all. Moreover, if models that emit a lot of particles make a player's FPS tank, chances are that they have already lowered their graphics settings, so this would make no difference whatsoever.
So please, ArenaNet. Let us have cute little birbs and giant stompy bunnies.
TL;DR: Miniature and Embiggening tonics should work while mounted, since this was a thing in the past and was removed due to an unrelated bug.