FILL axis is either 0 or 1, unfilled or filled
wght axis controls the stroke weight between 100 and 700, thin to bold, it can also affect the overall size of the symbol
GRAD axis is a more granular change to the thickness of the symbol between -25 and 200, it has a smaller impact on the overall size. A low grade is often used for a light symbol on a dark background
opsz axis is used to improve readability at different sizes, it does not change the size of the icon
font-size applies to these fonts, the default is 24px
<link rel="stylesheet" href="https://fonts.googleapis.com/css2?family=Material+Symbols+Outlined:opsz,wght,FILL,GRAD@20..48,100..700,0..1,-50..200&icon_names=add_circle"/>
<link rel="stylesheet" href="https://fonts.googleapis.com/css2?family=Material+Symbols+Outlined:opsz,wght,FILL,GRAD@40,400,0,200&icon_names=add_circle"/>