I write long copy plain vanilla pages without templates…

When I highlight a sub head feature to apply the h2 or h3 tag for styling, it changes the size of the surrounding text too…sometimes the entire text all that I have typed. not sure why this happens.and it is a problem in wordpress too. 

also it looks like it adjusts the space between rows. If you have any help for this, it would save a lot of time and frustation as I want to try dozens of letters with this type of formatting