What was actually wrong

Your pages had two faults at the same time. The text was too small, and the lines of text ran too far across the screen. Put those together and reading gets tiring fast. Your eye travels a long way to reach the end of a line, then has to hunt back across the page to find where the next one starts. Miss by a line and you read the same sentence twice without noticing.

The reason nobody flagged it is that it doesn't look broken. Small text in a wide column looks like a perfectly ordinary web page. It just quietly wears the reader down, and they leave a bit sooner than they would have. That's the whole problem, and that's why it was invisible.

How to read the pictures below

On the left is the page the way it was this morning. On the right is that same page as it is right now, live. Both pictures were taken at the same screen width, on the same page, so the only thing different between them is the thing we changed. Nothing else moved.

SEO Believer

The worst of the four. Lines ran 137 characters across, roughly twice as far as they should. They're 68 now, and the text is noticeably bigger. Instead of stretching wall to wall, the words sit in a proper column.

Cure Canine Disease

Almost the same story: 134 characters a line, now 71. This one matters more than most, because the people reading it have a sick dog and they're reading in a hurry. Bigger text, and the lines stop before they run off toward the edge.

Taste Boulder

This one is the opposite problem, and it's worth saying so, because otherwise the pair looks like a mistake. The column here was already a sensible width. The fault was that the text was tiny, the smallest on any site you own. So the two pictures look similar in shape, and the difference you're looking for is the size of the words, not the width of the block. Lines went from 90 characters to 69.

TrainerBooked

A milder version of the same fault: 103 characters a line, now 71. The column wasn't especially wide here, but the text was small enough that the lines still ran long.

Everything fixed today

Ten sites, all measured the same way: how many characters fit on one line of body text before, and how many fit now. Smaller is better here, up to a point, and that point is the last row of this page.