Why is the space so big/wide in the Japanese IME compared to the English IME?

Question

And is there a way to type a narrower, English style space while in the Japanese IME? (Like a key combination?)

Answer 1

As you mentioned in your own comment, a half-width space can be typed by Shift+Space. To answer your question about full-width space in your comment:

Since the Japanese characters are in squares with same width (as oppose to the English letters with different width), it would be neat if all characters are written in the same width. Since spaces are usually used for new paragraph line head indent, full-width space can be used to indent the first line neatly, like this:

　今日は２０１８年１０月７日です。天気がいいから、散歩しましょう。今日は２０１８年１０月７日です。天気がいいから、散歩しましょう。今日は２０１８年１０月７日です。天気がいいから、散歩しましょう。

　今日は２０１８年１０月７日です。天気がいいから、散歩しましょう。今日は２０１８年１０月７日です。天気がいいから、散歩しましょう。今日は２０１８年１０月７日です。天気がいいから、散歩しましょう。

This way, the characters are neatly aligned vertically character-by-character.

Answer 2

It may wider compared to the ASCII space, but it is the same width of a full-width character.

For example if you write several Japanese characters with space between them, the space has the same width as the character, so it is not wider:
あ　い　う　え　お