I have reset Windows 7 to default 100% and tested what happens in many programs. They all result in teeny-weeny fonts, including Opera 24 pages that I load, menu choices, etc. HOWEVER, the bookmarks bar and tabs are all the size in 100% setting that it used to be in Opera 23 with Windows set to 125%. Opera is the only program of a couple dozen that I tested, including MSIE, that does this.
As far as I can tell from the forum link provided by christoph142, the complaints were that Chrome overrode the Windows user zoom setting of 125% and reduced it to 100%. It was an issue of Chrome not using the user's Windows setting. I don't know about Chrome now. All I know is that Opera 24 is still overriding my Windows setting of 125% for the bookmark bar and tab sizes. I can reset the webpage zoom size within Opera, and it seems to respond to this about 75% of the time. Only slightly annoying that I have to do this reset every time I enter Opera 24 and several times during a session. Previously, 14 of my 15 bookmark folder tabs displayed on the bookmark bar. Now under Opera 24 only 11 tabs show on the bookmark bar (the rest require hitting the double arrow icon on the right side of the bar). I don't find any way to get the size down to where it is supposed to be, following my Windows setting of 125% and not something larger.