My impression is that the two samples are not using the same font. Please try to put a text with characters from both sets and see what happens.
Also try not to customize the font size
and even the font face
.
I also suspect that the text rendering engine may had overridden some of your changes due to the text length. iOS text rendering may try to change the font size of letter spacing if the text does not fit the control. So make some tests with shorter texts.
BTW, I think that you were mean to say that you want bigger font size for Japanese not smaller. In the screenshots the Japanese text is already too small to be properly read by anyone.