Like ....are G-Maxes meant to be a mega-like reaction to the dynamax energy and then also they do the projection to show them as huge? I suppose that tracks with how the animation plays out (iirc they're still sent out starting small-but-transformed then grow to full size) but its still pretty weird.
I'm not sure anyone at TPCI or GF has a actual coherent idea of what Dyna/Gigamax is...
And that's exactly the issue. They wanted the kaiju aesthetic but didn't actually want the Pokemon growing big so came up with a hand wave explanation you're expect to accept and not think about it further (despite making the region's professors area of study the Dynamax phenomenon so should be better explained...).
I'm going to assume the Pokemon do change form. However, due to the energy needing to access this form is also the energy that creates a large energy projection of the Pokemon, as far as we know you can't separate the Gigantamax form change with the large projection. Infact, with how some Gigantamax dex entries are written, it seems like Gigantamax Pokemon have a much closer connection to their projection selves than a Dynamax Pokemon.
To put my theory/headcanon into perspective, think that when a Pokemon Dynamaxes they're using the Wishing Star energy (WSE) to pretty much create a Power Ranger/Super Sentai zord of themselves, the projection. While the projection mimics their movements, the Pokemon and projection are two separate entities. This could be why Dynamax Pokemon only have access to the basic Max Moves, they may control the projection but not the WSE that created the projection so have to work within certain limits (the Ranger inside the zord is controlling it but the zord has its own abilities separate from the Ranger). As for why in the story some Pokemon go wild when suddenly Dynamax, well, how well do you think you'll do just suddenly placed inside a giant body of yourself, copying your exact movements and by your point of view you're still sensing things are "normal" (seeing from the eyes, smelling from the nose, hearing from the ears) but it's the projections eyes, nose, ears, etc. the sensory information is coming from. You'd probably be startled too, especially if you're a creature that relies on instincts more.
Gigantamax on the other hand is more complex. In addition to the SWE energy forming a projection around the Pokemon, it also is absorbed by the Pokemon thus changing their form (similar to Mega Evolution in that regard). However, now that the Pokemon has absorbed SWE within it, it now has a closer connection to the projection. It's not just the power of the SWE it absorbed that it's feeling, but also the power of the ENTIRE projection. The projection isn't mimicking its movements, they ARE its movements. And thus the Pokemon gains special attributes that it can then amplify via the projection (hence the Pokedex description) and even bend the SWE in different ways no other Pokemon can (hence the GMax Moves). Of course, with the Pokemon having so much energy now bonded to them some go kind of crazy or are unable to properly wield such power. Others may have the discipline/strength to handle the power but the physical changes that are now being amplified by the projection is unsustainable for a long period of time.