Mobile screen result

You have to find these rules:

@media only screen and (min-width:768px) and (max-width:989px) {
  body,
  body .avia-tooltip {
    font-size:80px
  }
  h1 {
    font-size:60px
  }
  h2 {
    font-size:50px
  }
  h3 {
    font-size:42px
  }
}
@media only screen and (min-width:480px) and (max-width:767px) {
  body,
  body .avia-tooltip {
    font-size:66px
  }
  h1 {
    font-size:46px
  }
  h2 {
    font-size:37px
  }
  h3 {
    font-size:29px
  }
}
@media only screen and (max-width:479px) {
  body,
  body .avia-tooltip {
    font-size:53px
  }
  h1 {
    font-size:38px
  }
  h2 {
    font-size:28px
  }
  h3 {
    font-size:20px
  }
}

This is a very unusual scaling.
First of all, the value for the base (body) is set very high.
Even if you take the aspect of wai aria into account, a body font-size of 53 ( below 479px) is gigantic. For desktop screen widths, this is 13px.
Here I would rather go higher, e.g. to 16px.

Because you have set a merging of the styles – i can not say where the rules comes from ( maybe it is a merging of W3total Cache ).
have a look if you have set on General Styling – Typography these values or in your quick css.