File size increases after editing TOC

In DT 4 I renamed a few entries in the Table of contents in a pdf, where each page has an entry and the names are the pages numbers. The saving of the pdf lasts about 1-2 minutes (while beachball is rotating) and the file size increases from 5.4 MB to 24 MB.
Is this normal behavior?

An increase in filesize when editing a PDF is nothing new in PDFKit enabled applications. However, the size and speed of saving can vary dramatically, depending on the document.
How many pages are in this PDF document?

How many pages does the PDF have?

Thank you!
It‘s a music score book with 151 pages.