We cannot shrink the size of a table which is then not displayed in Jira Service Management (Portal) in a proper way. Thus, users have to scroll all the way down to then scroll to the right. This is not user-friendly.

However, a workaround that maybe will work for you is to split this table into two tables and use Proforma's Layouts function and insert the table inside that layout. With this configuration you should be able to better organize your information, as in the example below:

In the customer portal, the result will be the one below: 

This workaround is suitable when the cell content is the same, the cell height
is the same size for each column.