What's happening is SSRS is being told to fit your report to the entirety of the available space in the viewer control; so columns are being stretched until the table consumes the entire page width.
You might try tweaking these values:
SizeToReportContent="True" Width="100%"
Additionally, ensure that your rendering command isn't forcing whole page width/100% fit/etc.
Lastly, it's probably not the issue you're facing, but good practice when you're concerned about maintaining specific sizes/formatting on a report would be to set the "allow growth" (and related properties) on the necessary controls in your report.