Taller text also needs to be slightly thicker. (If 48 pt text was the same thickness as 8pt text it would look incredibly skinny!). So you're simply seeing the way that your computer and monitor represent that slightly thicker text. (There are only a limited number of pixels on your screen. So when you enlarge text from one size to another the number of pixels actually displayed can only be an estimate of the exact number required). The ability of your computer's graphics card to display fine detail accurately is also relevant.
It might also be worth checking that your monitor is set up correctly. Go here:
https://reeddesign.co.uk/testcard.htm
You should be able to see all of the numbers, from 0 to 10, clearly on both of the relevant lines. If you can't, follow the instructions on that page to fix the problem.