This usually happens when you select the "Inline local files (.js and .css)" option during publishing and is also affected by the CSS Prefix options for WebKit and Mozilla. What's happening is that your css is getting much larger because of all the prefixes and all the components you use in your banner are being bundled into the main index.html file.
Often I just end up not checking the Inline local files option and that gets me below the 75k threshold on Studio, but leaves a lot of files in your project. If you don't like that, you can publish a version locally, and take all the CSS out of your index.html and paste it into a style.css that you link inside the index.html file. Then zip up that local copy and drop it into studio manually.
It would be ideal if they added a box that let you split all the CSS into a separate file since that is usually what actually has the bulk of the file size with the way GWD builds its animations out. I'd also really like to see an option that bundles all CSS and all JS into separate files so that the folder structure of the published project had an index.html with just the markup, and then a style.css and main.js