When merging documents containing the same embedded font every time a document is merged the same font is repeatedly embedded in the merged document. This leads to very large files.
A possible workaround before this feature is developed could be to use the PdfProcessing model to import and merge the documents. After the documents are merged to iterate the document`s content in order to cache the fonts using their name and if there are duplicated names to set the already cached one (check the attached project).
As a side note, the suggested workaround doesn't cover all possible cases (e.g. if there are fonts with a common name but different font sets (modified fonts or font subsets) this may lead to missing/different characters in the produced document). So in order to use this workaround, you will need to ensure the font names are unique for every different font.