edit PDF text with Illustrator - keeping the file size

Hi Anyone who could help me,

Is a bit tricky question, hopefully I manage to explain it correctly and that someone can help me.

I got 3 PDFs files, all this this files hade the same size file; 1.4 MB

I only needed to change one thing; the recent Text to another text. I tried to edit on the Acrobat but it didn't manage to select to the text I wanted to edit. So I transfer it to illustrator and manage to change it easy. Now, when it was time to save as to pdf. The pdf size file changed, I only change a text which was 3 sentences and in original had 3 sentence. Than I changed all of this 3 pdf files and all of than where different size file. I open them on acrobat and tried out optimize, reduce .. but I never manage to succeed the 3 files be on the same size file like I got it in original. How does this work? how did the other person manage to do this before? (MacBook Pro computer).

Thank you!!