It's hard to say without seeing your original HTML and resulting PDF. Everything beyond simple text can increase the file size significantly, but the typical culprit for a large PDF is an embedded font. When used, an embedded font adds the full font file because DOMPDF does not currently support font subsetting. Can you share the HTML and PDF?
Also, just FYI, this question would be more appropriate for the
dompdf support group. dompdf-dev is intended more for developer discussion of development of the DOMPDF project.