I'm not certain what the answer is without seeing your workbook but I found this post that might be of help. If this does not help and you are able to, please share your workbook.
Thanks and best,
It would help to hack up an example that you can share. Make a 100-row data source in excel. Build the sheet, the dashboard, the container. Then swap data sources with a 10-row excel file. See what happens.
In my opinion, if the 10 rows fit in the container, the sheet shouldn't shrink and still have a scroll bar. If that still happens, I would consider it a bug and report it to Tableau Support.