I mean, if it was the right size following you, it either couldn’t follow you on land basically at all, would be impossible to see around, and / or be clipping through everything. The battle thing is a heck of a lot more solid - you could argue that as a matter of arranging the Pokémon in a way that allows big boys - but it's still not a matter of just plugging it in.
The big limiting factor is twofold:
One, the trainer behind the pokemon is set at a fixed distance. The Pokemon have to be made to fit within a certain radius to work. This radius is why some Dynamax pokemon are larger than their Gigantamax counterparts - they're tall, thin Pokemon. Wailord is much longer than he is tall, so his size is constrained rather tightly.
You could have wailord occupy the Gigantamax spot by default, maybe? It wouldn't be perfect, but it'd be one option.
The other thing to keep in mind is that the way the Gamecube games animate their attacks and stuff is fundamentally different.
No, seriously. Watch.
Every single move is split up into two parts:
1: Attacking pokemon does the attack animation.
2: The camera cuts to the defender.
3: Defending Pokemon does the hit animation.
There's no continuity between the shots. This allows them to avoid any issues with pokemon size, but also means that even
some simple animations are flat-out impossible. It's also part of why the animation is slower - they can't show both the attack and the effect at the same time.
I actually just heard a great theory regarding a slightly earlier discussion - why Palkia is Water-type. To be clear, I'm building off the earlier theory that they represent the three states of matter - Solid Steel, Gaseous Ghost, and Liquid Water.
So, Dialga is Steel-type, and it's easy to make a connection between metals and timepieces. Giratina is ghost type, and it's easy to make connections between Giratina's gaseous theme, design, etc. But why is it that Palkia - a creature designed to represent space - associated with Water?
There's one theory that it comes from the tale of Archimedes - the Eureka moment. To learn the volume of a complex shape like a crown, Archimedes found that if you submerged the object in water, you could measure the displaced water to determine the volume. In other words, if Dialga's steel type is from tools for measuring time, Palkia's water type is from the first tool for measuring volume and space.
