In the Wikibooks/TeX article on fonts, it is stated that
\Hugecommands are often enough for most contents. These are fixed sizes however. In most document processors, you can usually choose any size for any font. This is because the characters actually get magnified. If it usually looks correct for medium sizes, it will look odd at extreme sizes because of an unbalanced thickness. In TeX it is possible to change the magnification of anything, but highly discouraged for the aforementioned reason. Changing the font size is made by changing the font file. Yes, there is a file for every size:
cmr10for Computer Modern Roman 10pt,
cmr12for Computer Modern Roman 12pt, etc. This ensure the characters are correctly balanced and remain readable at all defined sizes.
I do not know what exactly they mean by the "unbalanced thickness" (a side question: what is meant by this?). However, if I use
fontspec and a system font in either XeLaTeX or LuaLaTeX, there is --- as far as I know --- no file for every font size. So does
fontspec produce worse result for large fonts because of this? And finally, my main question: Is it still discouraged to use
\fontsize and arbitrary font sizes when using