Here is how I think it should work:
--force-device-scale-factor=1 should correspond to DPI 100%
--force-device-scale-factor=1.25 should correspond to DPI 125%
It should also be possible to change the scale factor from Opera settings. The default could follow the DPI in the OS, but it must be easy for the user to change it.
With this release of Opera I would use scale factor of 0.8 to get the size the previous Opera version had, but scale factors below 1.0 don't work.
A quick fix would be to allow a zoom level of 80% (the current options are ...75, 90, 100...). This would still leave the GUI elements big, but at least the pages would look the same as before and as with other browsers with zoom level of 100%. For example in IE the default zoom level is 125% if the DPI is 125%, but it is very easy to change that to 100% to get the desired page size.
The current version of Opera would show a webpage at 100% zoom level differently from other browsers with the same zoom level. I think this could bug some web developers.
You have some very nice features in Opera and I would like to continue to use it, but I must be able to view web pages at the size which is the same as 100% zoom of the previous stable Opera version. I think that exposing the scale factor in Opera settings would be a suitable solution for everyone.