You don't disclose what you are putting in your Page Copy to make it so large
If JavaScript there is another field that it can be placed, you will see it in the code editor usually next to the file with the Page Copy.
You can also include JavaScript in the Basic and Multi Step forms (use the Portal / PowerPages Management app), and from Web Files
Similarly there are additional fields for CSS and can be included from Web Files.
HTML and Liquid have more limited places but you can also create a Custom Web Template and include it in there, Web Templates can also have JavaScript and CSS also - the Web Page you see is a combination of what is in the Web Template, Web Page (e.g. Page Copy), and the Forms you are using.