The Japanese sentences that we see on smartphones and personal computers have an aspect ratio of hiragana / katakana and kanji that is almost a square, but alphanumeric characters have an aspect ratio of 2 to 1, basically Mixed characters. There are various theories on how this happened, but one of the reasons is the apparent impression that "2020" is cleaner than "2020".
There is also a viewpoint of the amount of information when expressing characters. There are more than a hundred different types of alphanumeric characters, including uppercase and lowercase, but Japanese has many character types, and there are more than 2,000 types of common kanji alone. Alphanumeric characters can be represented by 1 byte per character (0 to 255), but Japanese requires 2 bytes or more per character.
In the days when computers were not high-performance enough to handle many kanji, it was not possible to handle katakana by adding to the character definition (character code) called "ASCII" that can represent alphanumeric characters with 7 bits (0 to 128) It is considered that the character code established is "JIS X 0201".
Eventually, the character code "JIS X 0208" that can handle hiragana and kanji appeared, and it became necessary to mix and display ASCII and JIS X 0201. JIS X 0208 characters are square, ASCII / JIS X 0201 are generally displayed / printed with the same height but half the width, as a result, JIS X 0208 characters are “ full-width '', ASCII / JIS X 0201 characters are now called "half-width". Because it is katakana included in JIS X 0201, which is displayed in half the width compared to JIS X 0208, it is "half-width kana".
The reason why we talked about this long was that iOS 13 made it easy to enter half-width kana characters. After typing the characters before conversion as usual, tap "∨" at the right end of the keyboard to display the input candidates, and there should be one-byte kana at the end. It is a half-width Kana character defined in UTF-8 instead of JIS X 0201 mentioned above, but the display width is half of Kana / Kanji. From now on, there is no need to search the web or use the app to enter half-width kana.
Easy explanation of operation procedure