Only if you've told it to. My Firefox settings have "Minimum Font Size" set to "None". Perhaps scaling up to 12px is a default? (Edit: Also, are you sure you're not thinking of 12pt? IIRC, points are DPI-independent units and (AIUI) the traditional way of specifying font sizes in computerized typography.)
Despite my age, I still have eyes that are good enough to easily read the font sizes you're complaining about. A hugely important part of a User Agent is that it provide overrides for site design choices that the Agent's user has decided will benefit them. It's a good thing that UAs let folks like you choose a minimum-possible font size. It's an equally good thing that UAs let folks like me choose to see the choices that designer made that others criticize.