Why did Jet Jaguar Grow in Size

Jet Jaguar grew because of Plot Convenience (same goes for his appearance in Godzilla vs Megalon), he needed to grow so he "programmed himself to change sizes".

Jet Jaguar in a nutshell:

SasquaDash

it has scientific basis like the rest of the plot, it’s a science driven story, not a action or character driven one

HinikunaGoji

I know that the show is more focused on science than action, my issue is that they don't really explain Jet's growth in the show, it just sort of happens. Godzilla vs Megalon at least gave some explanation for his growth (even if it was just a throw away line). Again, I get that the show is more based on science, but I feel like we shouldn't need to do hours of scientific research in order to understand why Jet Jaguar suddenly decided to change size.

SasquaDash

It took me two minutes, and another thing is it’s definitely not trying to cater to an American audience. It’s full of references only Japanese viewers would get and there’s a lack of marketing in America as well. It didn’t take me hours to research I just knew it already and didn’t make a connection 

I think leaving the questions open ended leaves room for more story opportunities. I don’t think it’s as much of a problem if the story isn’t done.

They also set up Jet Jaguar's change in dimension an episode earlier when they explained the archetype dust was changing the dimensional fabric of reality. Being a singular point allows for creatures that can not exist in our world to exist, including Jet Jaguar's dimensions. That one I got out of the gate.

Jet Jaguar coded himself to change size 

Mothra!

I was talking more about the actual science behind it

Jet Jaguar will definitely change its size.