This is because as new measures and resulting pages are added, the score formatting takes on settings found under Document Menu > Page Format > Score. In this case, the music is much larger than before because of those particular settings.

A quick fix:

:

√ Go to Page Layout Tool and under the menu make sure "Update Page Format for Score" is checked.

√ With the Resize Tool "%", go to the last correctly sized page and click in the corner of the page. Hit OK.

√ Again with the Resize Tool, on the same page, click in the middle of the system, not on a measure but in the white space in between staves. Hit OK.

Problem solved.