I just ran both reports for the same time period looking at a specific page and the visits are off by 50% in Full Path Report.
I am choosing specific page in both the Page Report and when I choose page in "Showing paths containing" in Full Path Report.
The Full paths report, doesn't actually take into account all visits. The report only analyzes on visits which had a maximum of 8 pages viewed. Any visits having more than that is not recorded for the full paths report.
Here is the documentation for it: https://marketing.adobe.com/resources/help/en_US/reference/reports_fullpaths.html
This could be very well why you are having lower visits count for a page in these full paths report.