Legendary Godzilla is in a literally HUGE advantage because of size and stuff, but what if Zilla Junior climbs on his back (since he is so agile, smart and fast) and breaks Godzillas neck?
Would Zilla Jr. be smart enough for that? I think he eventually would. I think if Dr. Nick would tell him to, he would understand and do it.
At least he should have the strenght to do so as we saw him lifting a Giant rock like a monkey and throwing it at Crustaceosaurus-Rex... Just his High speed digging would mean tons of bizeps strenght...
And his arms are long and unhandy, not like for example Angirus's legs which are short and perfectly adapted and shaped for digging, which means Zilla Juniors Arms despite being not well build for digging must have incredible strenght to dig in that high amount of speed...
I think Zilla Junior could win this despite being doubble as small... He moved much faster than Legendary Gozilla did in the movie, and he is much smarter than the Mutos and i think he is much smarter than Legendary Godzilla.
An smaller Jaguar also manages to defeat an larger, stronger but less agile Caiman...
You know what?
I go with Zilla Junior because of intelligence and agility, he is not incredibly stupid like that zilla in final wars that he would charge mindlessly at such GIGANTIC opponent...