Hello,
sorry that this thread remained unanswered for so long.
According to your numbers, switching to printing did help: the January PDF went down from 52.7 MB to 25.4 MB. The remaining difference to December is usually caused by the content, for example more or larger photos in the magazine, since images make up most of the size of such a PDF.
To make the PDF smaller, compress it afterwards with Compress PDF in the PDF24 Toolbox. Lower values for DPI and Image quality give smaller files. This also works online: https://tools.pdf24.org/en/compress-pdf