there isnt really any need to scale the font every tick...thats 60 changes per second...no one can see that kind of changes..
I suggest only changing it when you need to..not every tick
The thing is with Font size is that the border box of the text object determines whether the text appears at whatever size..do you understand ?
so if your text object border size is too small when you enlarge the font it will not appear inside the box..
you would need to scale the box size with the font size
hope that clears things up