Highlighter thickness in Preview doubled overnight

I am using Preview 8.1 (877.7) on El Capitan (10.11.6).

I have been reading and highlighting a document. Overnight the highlighter thickness doubled, so that it highlights the nearby lines of text, too. In the attached image you can see such change.

I tried:

• re-exporting the PDF;
• restarting the computer;
• restarting Preview;

but none of this worked to restore the previous highlighter size.

How do I return the highlighter to normal size?

Thank you for your help