Anki's default template lets card content stretch across the full width of the screen. That's fine on a phone, but on a monitor a line of text gets so long that your eyes struggle to track it from one end to the other.
In ankieditor, click an empty area of the card to select the whole card, and a "Max card width (leave blank for no limit)" input appears. Setting it to somewhere around 500px pulls the content toward the center on any screen, keeping it readable. It ends up feeling roughly like the width of a page in a printed book.
For the same reason, bumping line height up to around 1.5 helps too — it makes it easier for your eyes to take in a whole sentence at once while you're studying.