When converting a PDF to PDF/a you may notice that the file size becomes bloated, for example 18KB -> 448KB.
There are 2 reasons for the increase in size when we select the “PDFA-1b” conversion:
1) extra fonts which get embedded in the resulting PDF/a document
2) the color scheme chosen in the “PDFA-1b” setting. If you select CMYK then the size gets increased drastically. But if you choose RGB then the difference in size is negligible, assuming there are no fonts to be embedded.
To get the smallest file-size during PDF to PDF/a conversion, you should use the RGB conversion options.