I don't do it like that Chris.

Create site and then create variant.

It can be confusing, you can create text by just clicking in the text tool anywhere on the page = a Text Line
or
create text by clicking & dragging the mouse horizontally in the text tool= a Text Column
or
create text by clicking and dragging right & down in the text tool = a Text Area

Sorry if I'm teaching you how to suck eggs.

I tend to use Text Areas which show as red dotted lines around the text if you're in the Text tool.
So you've created the Main site & then created a Mobile Variant.

The Main site will be fine.
The Mobile site may need the text container resized, for example it's wider that the mobile screen.
So click the top right corner in the text tool and narrow the width down from the right to fit page width.

Having done this there almost always won't be enough height in the text area and you'll need to lengthen the area down.

I recommend using push with these text areas or they can easily overlay other page content.

Now, in the Mobile variant you can select all (or part) of the text content and change the font, the size & colour.

Then the style (say normal text) will have a plus sign appended to the end (normal text +)

This is the time you are required to select the style and hit Update style in this variant. The style remains as per original in the Main site but takes on a different style in the Mobile site.

In your example:
Create site and then create variant
Select text box and click Update Normal Text In This Variant
Edit font size (either dragging selection handles or via font size drop down)
Size alters on both variants
Is basically arse about tit.

You need to edit the mobile font first by changing size, font, etc then select Update Normal Text In This Variant.