On a copy of your file try this...

1. Choose the Resize (%) Tool
2. Right-click in a measure of the last system (on p. 38) that is the size you want
3. Choose Resize System
4. A panel will appear with the specs for that system pre-entered
5. Select System [whatever] through end of piece
6. Click OK

Because your description does not make clear whether the page size has changed or the system size has changed, I am making an educated guess here, which is why I strongly suggest you try this on a copy of your file.

… suddenly after page 38 everything seems bigger, the notes, the staffs etc and it does not fit anymore in one page …

Probably the culprit can be found here:

Document menu > Page Format > Score…

The settings in {Page Format for Score} are used when new systems/pages are created.

Hence the problem appears after Page 38.

You should change the settings in {Page Format for Score} so that they match the formatting settings on the first 38 pages.

When you have done so, you can fix the layout via Redefine Pages:

Page Layout Tool.

Page Layout menu > Redefine Pages…