Why some webpages do not get proportionally larger
Not all webpages take their sizing info from the DPI resolution setting from Windows. Sometimes, I think, they use the actual screen resolution (the native setting). For those pages, if you have a high resolution screen, the text will be pretty small - or, at least, highly detailed. Yahoo Email, new Email style, is like that.
I have a laptop computer with a 1920x1200 15" screen, you want to see some very small text!! That is the same resolution as my 26" LCD screen.
1920x1200 resolution on a 26" screen is about 90 pixels per inch, but on a 15" screen it is nearly 150 pixels per inch.
With my 26" screen I do not mind the reading of the Yahoo Email, but on that laptop it gets pretty small. So, on the laptop, I generally temporarily switch over to Yahoo Mail Classic which is not as affected by really fine screen resolution. Maybe you have some similar options for what you are doing.