https://github.com/fabricjs/fabric.js/blob/master/src/shapes/text.class.js#L210
_fontSizeFraction: 0.222,
_fontSizeMult: 1.13,
Is there any clue how they come from ? I found they are very useful to measure the actual text size.
_fontSizeFraction
is approximately equal to 2/9, but my impression is that this value would be used to decrease the default size of the font.
Text Line proportion to font Size (in pixels)
_fontSizeMult
increases values by 13%. It takes the font height, increases it by 13% to give you the total line height including line-spacing.
These variables look like defaults and were likely chosen by the developer based on personal preference.
They work for finding the text size because the text size is likely defined by these variables.