The fill style is set up to tile to fill without a target scale. This basically means it is mapped pixel to pixel to determine how many times it repeats. The tiling isn't a huge issue on the screen, but when exporting to a high resolution, there are many more pixels, so it will tile many more times.

Click the fill style indicator on the status bar, go to the bitmap files tab, find the fill (CA22_Starfileld 06 Bitmap I believe), and place a check mark in the "Scaled" option and change the widht and height values to 200.

(If you don't like how often it repeats, you can go back in and change these scale numbers. The higher they are, the less often it repeats, but if the numbers are too high it may cause the image to be stretched further than it's actual quality, resulting in pixelation.)

Just to throw a cog into the wrenches here. All atlas maps are always exported without the grid, because that often makes problems for people using VTT's, which is one of the main reasons to download the rendered image version. But then, referring to grid size becomes meaningless. So atlas maps do need an actual scale bar of some sort, but a tiny one is more than enough for maps such as these, doesn't need to be any larger than a single grid square.

To make zoom-friendly glows for text, use the Percentage of View Width effect scaling option. This means the width will be relative to the current view, instead of the map. Of course, this also requires a very different size setting for the range of the glow.

Nice. Bergenhus is an important historic site in my home town. Here's an illustration and some text of the real thing. Today, it serves as a museum and an outdoor concert arena.

Ralf had a blog series about this:

He also has a live mapping session with the topic:

If you own the Tome, there is also a tutorial on this starting on page 154