Let’s fix font size
One of the factors in the font bounding box is character accents. We don't use them much in the U.S., but other countries do. Looking at Fira Code, for example, it seems the bounding box is unnecessarily tall for a capital A. But when you look at the accented capital A, it is about right (it has a little extra padding). Still, there is some inconsistency. For the example, the "Latin Small Letter L With Acute" (ĺ) in Fira Code breaks out of the bounding box (whereas the same accent on the capital L doesn't). The same with descenders and their accents. The bottom boundary accommodates most of them, but some hang just a bit below. The "Box Drawings" glyphs included in Fira Code extend well outside the bounding box horizontally and vertically (but are kiss-fit on the left and right edges).As a designer, I agree with your basic point, that more consistency is needed. But it's worth nothing that the same issues affect traditional print design. A 10 pt font in Adobe InDesign, whether in an inch, point, or mm based layout, will vary in size from one font to the next. Their cap heights and x-heights will be different, sometimes dramatically. However, the one thing that will be the same is their leading (baseline spacing). That would be a good start for a reformation with regard to browser displays.For now, it is probably a case where we can blame the browser devs or standards organizations, but they will in turn blame the specs of the source fonts, which are in fact inconsistent as I said above.