You are not alone. I also have the plastic Japanese infantry. I know what you are talking about. In fact, this is one of the main reasons I made a comment about this sometime ago. Here is a link:
The problem is not the height of the figures, but its the over-heroic proportions (over-exaggerated girth and features). Because when it comes to height, the Japanese infantry are just fine when compared to the older and newer plastic box sets, but as you have noticed, when compared to their newer plastic box sets, well, they look thinner. That is because they do not have the over-heroic proportions. The newer plastic box sets look, well, like very nicely detailed Stay-Puff marshmallow soldiers.
We will have to wait until they release a new plastic Japanese infantry box set to get that Stay-Puff marshmallow soldier look.
Take a look at the new plastic British infantry set that was just released. Very nice figures, but they have that Stay-Puff marshmallow soldier look. More then the older plastic British infantry set. I was hoping we would be able to use the arms from the older set with this newer set, but we can’t because if you do, you get something like Arnold Schwarzenegger bodies with skeleton arms.
Long story short. The plastic Japanese infantry have better anatomically correct proportions for their height while the newer plastic box sets are becoming more and more heroically over-sized. More details in the post linked above.
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