That's not a lot of spacing, but that's not zero. If your first paragraph were longer, it would wrap, and the lines of your first paragraph would have less space between them than there is between the two paragraphs.

I don't know why the CSS authors of the default Lemmy theme have chosen such a small paragraph spacing. You're not the first person to complain about it. It's pretty bad.

In my Lemmy client, Thunder, your post looks great. Anything you do to increase the space will just make it look bad for people like me. My suggestion is to install a style plugin and write a tiny CSS override to increase the paragraph spacing so it looks good for you. Then all Lemmy posts will look good for you, and everyone else using other views of Lemmy will also see normal paragraphs. (And maybe someday the Lemmy devs will fix it because people keep complaining about it.)

If you want help setting up a personal stylesheet, let me know, and I'll help you get it set up.