[Shurikenjin]

They arent smaller really, its more that they are different. Ever since 2010's redo of the original megazord, Bandai of America created a zord builder port which meant that zrds from different series using this system could interact with one another and Japan didnt have that. So, lets say you have that red t-rex from Dino Charge, both the Japanese and US version have the gimick of replacing forearms BUT with the US version, you could pull off th entire arm with shoulder as well and place it on the legacy megazord if you wanted to. Kids loved that so Bandai of America have been making every megazord sicne then have his compatibility along with other acessories (like bikes, or cannons). ut the thing is that if all megazords have to be combatible, they must all be roughly the same size. And the same size also helps keep things at the same price points. Like that every year, a mother or grandfather can go to a toysrus and expect to pay the same price as last year (or about the same). Kinda like how they do it with Transformers or marvel figures where all the standard sizes hae the same price point.

But in Japan they dont work that way.
1. They dont have standard sizes, so one series can have big zords and aother have small ones.
2. They dont need to all be compatible with eachother.

So you end up with all kinds of different sizes when you look at the US vs Japanese zords. They can be bigger (like the Japanese versions of Dino Charge), or be very small (like the cubes of last year in japan) or be the same size as the US standard size (the Japanese version of Ninja Steel).