Current line text font size gets increased while typing in Xcode

I had a similar problem but I am not sure is exactly the same. Did you try to change this settings?

Xcode preferences > Fonts & Colors Your chosen theme will have a "current line" option just under the font. Can't remember exactly but I think you can find something there.

Solution:

This happens when you change font size for one particular element but the other editor's elements are using the previous/default font size.

If you want to change the font size either press cmd + or in preferences -> Fonts and Colors choose your theme and in the right pane select all the elements (press cmd a) and then change the font size.

See this SO answer