This is one instance where I mostly agree with you. I don’t have a major issue with the contrast between the font and its background in the new design. Not everything should “pop” on the interface, IMO. However, something is definitely going on with the font because the smaller size font in the new design is noticeably blurrier.
Also, given the fact that these different Track Inspector sections will most often be hidden thus requiring repeated clicking on the arrows to expand or collapse a given section, it was better when these expandable/collapsable section labels were larger to provide a larger affordance.
That said, unless you hide some of these different sections, you will need to scroll up and down frequently on a smaller display. So I understand what Steinberg was going for by making these section labels smaller.