Font Resize makes letters small at first

Hi,

I have a problem with font resize function. When I click on resize button, it makes some letters smaller at first and on second click it increases the size. It’s mostly the headings that doesn’t work correctly.

Here is how it look before click – https://prnt.sc/bPL27ZEbkk2H

After 1 click – https://prnt.sc/SCVovx1YS6Bs

The page I need help with: [log in to see the link]