I am having a problem with the font size of labels changing in the layout window. Any font size above 12pt saves and prints okay but if I drop to 12pt, the font size dramatically reduces in size and is illegible. Also the increase in text size above 12pt does not seem accurate. There is only a very slight increase between point size. The only solution I see is to make my text boxes larger and only work with > 12pt font size.
I’m using the latest QGIS version (3.28.2).
I have also experienced this issue, and my solution involves making changes in the main map Layer Styling, where I change the font size from millimeters to Meters at Scale. This seems to "lock in" the font size so that it appears the same in the layout window (see example below). Don't forget to refresh the layout view after you make Layer Styling changes.
I don't know why the default millimeters setting acts as you have described; perhaps others with more knowledge can add their thoughts.
Finally, I have also used Meters at Scale with line widths, marker sizes, etc.
On the same line as the previous answer, I have found that the best way to avoid these differences in font size is to always use Map Units, as you are making sure that the font locks in to any change you apply to your map.
However, it would be helpful to see some examples of your project, so we can perhaps identify what's going on.
