Page set to A3 but the document is larger than that

I transformed a pdf to an editable .odt format and got very large margins.
I thought I could deal with it very quickly, so I opened Format > Page and changed margins but with no effect. Then I made sure the format of the page was set to A3, and it was also correct.
What can I do to cut the white space?

Page set to A3 yet the document is still larger

Thank you for answers, even though to you it may be a silly problem.